diff --git a/crates/red_knot_python_semantic/resources/mdtest/comparison/integers.md b/crates/red_knot_python_semantic/resources/mdtest/comparison/integers.md
index 748b94eda3..b576ce318d 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/comparison/integers.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/comparison/integers.md
@@ -19,7 +19,8 @@ reveal_type(1 <= "" and 0 < 1)  # revealed: @Todo | Literal[True]
 
 ```py
 # TODO: implement lookup of `__eq__` on typeshed `int` stub.
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 reveal_type(1 == int_instance())  # revealed: @Todo
 reveal_type(9 < int_instance())  # revealed: bool
diff --git a/crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md b/crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md
index d14bd93680..c80ee4d601 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md
@@ -59,7 +59,8 @@ reveal_type(c >= d)  # revealed: Literal[True]
 
 ```py
 def bool_instance() -> bool: ...
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 a = (bool_instance(),)
 b = (int_instance(),)
@@ -159,7 +160,8 @@ reveal_type(a >= a)  # revealed: @Todo
 "Membership Test Comparisons" refers to the operators `in` and `not in`.
 
 ```py
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 a = (1, 2)
 b = ((3, 4), (1, 2))
diff --git a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_elif_else.md b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_elif_else.md
new file mode 100644
index 0000000000..7ac16f4b8d
--- /dev/null
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_elif_else.md
@@ -0,0 +1,57 @@
+# Narrowing for conditionals with elif and else
+
+## Positive contributions become negative in elif-else blocks
+
+```py
+def int_instance() -> int:
+    return 42
+
+x = int_instance()
+
+if x == 1:
+    # cannot narrow; could be a subclass of `int`
+    reveal_type(x)  # revealed: int
+elif x == 2:
+    reveal_type(x)  # revealed: int & ~Literal[1]
+elif x != 3:
+    reveal_type(x)  # revealed: int & ~Literal[1] & ~Literal[2] & ~Literal[3]
+```
+
+## Positive contributions become negative in elif-else blocks, with simplification
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2 if bool_instance() else 3
+
+if x == 1:
+    # TODO should be Literal[1]
+    reveal_type(x)  # revealed: Literal[1, 2, 3]
+elif x == 2:
+    # TODO should be Literal[2]
+    reveal_type(x)  # revealed: Literal[2, 3]
+else:
+    reveal_type(x)  # revealed: Literal[3]
+```
+
+## Multiple negative contributions using elif, with simplification
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2 if bool_instance() else 3
+
+if x != 1:
+    reveal_type(x)  # revealed: Literal[2, 3]
+elif x != 2:
+    # TODO should be `Literal[1]`
+    reveal_type(x)  # revealed: Literal[1, 3]
+elif x == 3:
+    # TODO should be Never
+    reveal_type(x)  # revealed: Literal[1, 2, 3]
+else:
+    # TODO should be Never
+    reveal_type(x)  # revealed: Literal[1, 2]
+```
diff --git a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is.md b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is.md
index 8c042c75d0..b9cd22897d 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is.md
@@ -11,6 +11,8 @@ x = None if flag else 1
 
 if x is None:
     reveal_type(x)  # revealed: None
+else:
+    reveal_type(x)  # revealed: Literal[1]
 
 reveal_type(x)  # revealed: None | Literal[1]
 ```
@@ -30,6 +32,8 @@ y = x if flag else None
 
 if y is x:
     reveal_type(y)  # revealed: A
+else:
+    reveal_type(y)  # revealed: A | None
 
 reveal_type(y)  # revealed: A | None
 ```
@@ -50,4 +54,26 @@ reveal_type(y)  # revealed: bool
 if y is x is False:  # Interpreted as `(y is x) and (x is False)`
     reveal_type(x)  # revealed: Literal[False]
     reveal_type(y)  # revealed: bool
+else:
+    # The negation of the clause above is (y is not x) or (x is not False)
+    # So we can't narrow the type of x or y here, because each arm of the `or` could be true
+    reveal_type(x)  # revealed: bool
+    reveal_type(y)  # revealed: bool
+```
+
+## `is` in elif clause
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = None if bool_instance() else (1 if bool_instance() else True)
+
+reveal_type(x)  # revealed: None | Literal[1] | Literal[True]
+if x is None:
+    reveal_type(x)  # revealed: None
+elif x is True:
+    reveal_type(x)  # revealed: Literal[True]
+else:
+    reveal_type(x)  # revealed: Literal[1]
 ```
diff --git a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is_not.md b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is_not.md
index f7032d02af..f23bae8e1e 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is_not.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_is_not.md
@@ -13,6 +13,8 @@ x = None if flag else 1
 
 if x is not None:
     reveal_type(x)  # revealed: Literal[1]
+else:
+    reveal_type(x)  # revealed: None
 
 reveal_type(x)  # revealed: None | Literal[1]
 ```
@@ -29,6 +31,8 @@ reveal_type(x)  # revealed: bool
 
 if x is not False:
     reveal_type(x)  # revealed: Literal[True]
+else:
+    reveal_type(x)  # revealed: Literal[False]
 ```
 
 ## `is not` for non-singleton types
@@ -43,6 +47,27 @@ y = 345
 
 if x is not y:
     reveal_type(x)  # revealed: Literal[345]
+else:
+    reveal_type(x)  # revealed: Literal[345]
+```
+
+## `is not` for other types
+
+```py
+def bool_instance() -> bool:
+    return True
+
+class A: ...
+
+x = A()
+y = x if bool_instance() else None
+
+if y is not x:
+    reveal_type(y)  # revealed: A | None
+else:
+    reveal_type(y)  # revealed: A
+
+reveal_type(y)  # revealed: A | None
 ```
 
 ## `is not` in chained comparisons
@@ -63,4 +88,10 @@ reveal_type(y)  # revealed: bool
 if y is not x is not False:  # Interpreted as `(y is not x) and (x is not False)`
     reveal_type(x)  # revealed: Literal[True]
     reveal_type(y)  # revealed: bool
+else:
+    # The negation of the clause above is (y is x) or (x is False)
+    # So we can't narrow the type of x or y here, because each arm of the `or` could be true
+
+    reveal_type(x)  # revealed: bool
+    reveal_type(y)  # revealed: bool
 ```
diff --git a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_nested.md b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_nested.md
index 46492f8cca..cc0f79165e 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_nested.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_nested.md
@@ -3,7 +3,8 @@
 ## Multiple negative contributions
 
 ```py
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 x = int_instance()
 
@@ -27,3 +28,29 @@ if x != 1:
     if x != 2:
         reveal_type(x)  # revealed: Literal[3]
 ```
+
+## elif-else blocks
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2 if bool_instance() else 3
+
+if x != 1:
+    reveal_type(x)  # revealed: Literal[2, 3]
+    if x == 2:
+        # TODO should be `Literal[2]`
+        reveal_type(x)  # revealed: Literal[2, 3]
+    elif x == 3:
+        reveal_type(x)  # revealed: Literal[3]
+    else:
+        reveal_type(x)  # revealed: Never
+
+elif x != 2:
+    # TODO should be Literal[1]
+    reveal_type(x)  # revealed: Literal[1, 3]
+else:
+    # TODO should be Never
+    reveal_type(x)  # revealed: Literal[1, 2, 3]
+```
diff --git a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not_eq.md b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not_eq.md
index d0af94e9ea..3ad8ebcb68 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not_eq.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not_eq.md
@@ -11,6 +11,9 @@ x = None if flag else 1
 
 if x != None:
     reveal_type(x)  # revealed: Literal[1]
+else:
+    # TODO should be None
+    reveal_type(x)  # revealed: None | Literal[1]
 ```
 
 ## `!=` for other singleton types
@@ -24,6 +27,9 @@ x = True if flag else False
 
 if x != False:
     reveal_type(x)  # revealed: Literal[True]
+else:
+    # TODO should be Literal[False]
+    reveal_type(x)  # revealed: bool
 ```
 
 ## `x != y` where `y` is of literal type
@@ -54,6 +60,25 @@ C = A if flag else B
 
 if C != A:
     reveal_type(C)  # revealed: Literal[B]
+else:
+    # TODO should be Literal[A]
+    reveal_type(C)  # revealed: Literal[A, B]
+```
+
+## `x != y` where `y` has multiple single-valued options
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2
+y = 2 if bool_instance() else 3
+
+if x != y:
+    reveal_type(x)  # revealed: Literal[1, 2]
+else:
+    # TODO should be Literal[2]
+    reveal_type(x)  # revealed: Literal[1, 2]
 ```
 
 ## `!=` for non-single-valued types
@@ -74,3 +99,21 @@ y = int_instance()
 if x != y:
     reveal_type(x)  # revealed: int | None
 ```
+
+## Mix of single-valued and non-single-valued types
+
+```py
+def int_instance() -> int:
+    return 42
+
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else 2
+y = 2 if bool_instance() else int_instance()
+
+if x != y:
+    reveal_type(x)  # revealed: Literal[1, 2]
+else:
+    reveal_type(x)  # revealed: Literal[1, 2]
+```
diff --git a/crates/red_knot_python_semantic/resources/mdtest/narrow/isinstance.md b/crates/red_knot_python_semantic/resources/mdtest/narrow/isinstance.md
index 5b5d984107..4b07d5648a 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/narrow/isinstance.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/isinstance.md
@@ -40,6 +40,8 @@ x = 1 if flag else "a"
 
 if isinstance(x, (int, str)):
     reveal_type(x)  # revealed: Literal[1] | Literal["a"]
+else:
+    reveal_type(x)  # revealed: Never
 
 if isinstance(x, (int, bytes)):
     reveal_type(x)  # revealed: Literal[1]
@@ -51,6 +53,8 @@ if isinstance(x, (bytes, str)):
 # one of the possibilities:
 if isinstance(x, (int, object)):
     reveal_type(x)  # revealed: Literal[1] | Literal["a"]
+else:
+    reveal_type(x)  # revealed: Never
 
 y = 1 if flag1 else "a" if flag2 else b"b"
 if isinstance(y, (int, str)):
@@ -75,6 +79,8 @@ x = 1 if flag else "a"
 
 if isinstance(x, (bool, (bytes, int))):
     reveal_type(x)  # revealed: Literal[1]
+else:
+    reveal_type(x)  # revealed: Literal["a"]
 ```
 
 ## Class types
@@ -82,6 +88,7 @@ if isinstance(x, (bool, (bytes, int))):
 ```py
 class A: ...
 class B: ...
+class C: ...
 
 def get_object() -> object: ...
 
@@ -91,6 +98,16 @@ if isinstance(x, A):
     reveal_type(x)  # revealed: A
     if isinstance(x, B):
         reveal_type(x)  # revealed: A & B
+    else:
+        reveal_type(x)  # revealed: A & ~B
+
+if isinstance(x, (A, B)):
+    reveal_type(x)  # revealed: A | B
+elif isinstance(x, (A, C)):
+    reveal_type(x)  # revealed: C & ~A & ~B
+else:
+    # TODO: Should be simplified to ~A & ~B & ~C
+    reveal_type(x)  # revealed: object & ~A & ~B & ~C
 ```
 
 ## No narrowing for instances of `builtins.type`
diff --git a/crates/red_knot_python_semantic/resources/mdtest/subscript/bytes.md b/crates/red_knot_python_semantic/resources/mdtest/subscript/bytes.md
index bd807db36c..15330e4006 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/subscript/bytes.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/subscript/bytes.md
@@ -26,7 +26,8 @@ reveal_type(y)  # revealed: Unknown
 ## Function return
 
 ```py
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 a = b"abcde"[int_instance()]
 # TODO: Support overloads... Should be `bytes`
diff --git a/crates/red_knot_python_semantic/resources/mdtest/subscript/string.md b/crates/red_knot_python_semantic/resources/mdtest/subscript/string.md
index 586eb4dfda..6987a95a70 100644
--- a/crates/red_knot_python_semantic/resources/mdtest/subscript/string.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/subscript/string.md
@@ -23,7 +23,8 @@ reveal_type(b)  # revealed: Unknown
 ## Function return
 
 ```py
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 a = "abcde"[int_instance()]
 # TODO: Support overloads... Should be `str`
diff --git a/crates/red_knot_python_semantic/src/semantic_index/builder.rs b/crates/red_knot_python_semantic/src/semantic_index/builder.rs
index abbdd3f0c4..523682fcd3 100644
--- a/crates/red_knot_python_semantic/src/semantic_index/builder.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/builder.rs
@@ -27,7 +27,7 @@ use crate::semantic_index::use_def::{FlowSnapshot, UseDefMapBuilder};
 use crate::semantic_index::SemanticIndex;
 use crate::Db;
 
-use super::constraint::{Constraint, PatternConstraint};
+use super::constraint::{Constraint, ConstraintNode, PatternConstraint};
 use super::definition::{
     AssignmentKind, DefinitionCategory, ExceptHandlerDefinitionNodeRef,
     MatchPatternDefinitionNodeRef, WithItemDefinitionNodeRef,
@@ -243,12 +243,23 @@ impl<'db> SemanticIndexBuilder<'db> {
         definition
     }
 
-    fn add_expression_constraint(&mut self, constraint_node: &ast::Expr) -> Expression<'db> {
+    fn add_expression_constraint(&mut self, constraint_node: &ast::Expr) -> Constraint<'db> {
         let expression = self.add_standalone_expression(constraint_node);
-        self.current_use_def_map_mut()
-            .record_constraint(Constraint::Expression(expression));
+        let constraint = Constraint {
+            node: ConstraintNode::Expression(expression),
+            is_positive: true,
+        };
+        self.current_use_def_map_mut().record_constraint(constraint);
 
-        expression
+        constraint
+    }
+
+    fn add_negated_constraint(&mut self, constraint: Constraint<'db>) {
+        self.current_use_def_map_mut()
+            .record_constraint(Constraint {
+                node: constraint.node,
+                is_positive: false,
+            });
     }
 
     fn push_assignment(&mut self, assignment: CurrentAssignment<'db>) {
@@ -285,7 +296,10 @@ impl<'db> SemanticIndexBuilder<'db> {
             countme::Count::default(),
         );
         self.current_use_def_map_mut()
-            .record_constraint(Constraint::Pattern(pattern_constraint));
+            .record_constraint(Constraint {
+                node: ConstraintNode::Pattern(pattern_constraint),
+                is_positive: true,
+            });
         pattern_constraint
     }
 
@@ -639,7 +653,8 @@ where
             ast::Stmt::If(node) => {
                 self.visit_expr(&node.test);
                 let pre_if = self.flow_snapshot();
-                self.add_expression_constraint(&node.test);
+                let constraint = self.add_expression_constraint(&node.test);
+                let mut constraints = vec![constraint];
                 self.visit_body(&node.body);
                 let mut post_clauses: Vec<FlowSnapshot> = vec![];
                 for clause in &node.elif_else_clauses {
@@ -649,7 +664,14 @@ where
                     // we can only take an elif/else branch if none of the previous ones were
                     // taken, so the block entry state is always `pre_if`
                     self.flow_restore(pre_if.clone());
-                    self.visit_elif_else_clause(clause);
+                    for constraint in &constraints {
+                        self.add_negated_constraint(*constraint);
+                    }
+                    if let Some(elif_test) = &clause.test {
+                        self.visit_expr(elif_test);
+                        constraints.push(self.add_expression_constraint(elif_test));
+                    }
+                    self.visit_body(&clause.body);
                 }
                 for post_clause_state in post_clauses {
                     self.flow_merge(post_clause_state);
diff --git a/crates/red_knot_python_semantic/src/semantic_index/constraint.rs b/crates/red_knot_python_semantic/src/semantic_index/constraint.rs
index 9659d5f82f..44b542f0e9 100644
--- a/crates/red_knot_python_semantic/src/semantic_index/constraint.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/constraint.rs
@@ -7,7 +7,13 @@ use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::{FileScopeId, ScopeId};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub(crate) enum Constraint<'db> {
+pub(crate) struct Constraint<'db> {
+    pub(crate) node: ConstraintNode<'db>,
+    pub(crate) is_positive: bool,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(crate) enum ConstraintNode<'db> {
     Expression(Expression<'db>),
     Pattern(PatternConstraint<'db>),
 }
diff --git a/crates/red_knot_python_semantic/src/types.rs b/crates/red_knot_python_semantic/src/types.rs
index d445b694be..c932492f21 100644
--- a/crates/red_knot_python_semantic/src/types.rs
+++ b/crates/red_knot_python_semantic/src/types.rs
@@ -332,6 +332,11 @@ impl<'db> Type<'db> {
             .expect("Expected a Type::ModuleLiteral variant")
     }
 
+    #[must_use]
+    pub fn negate(&self, db: &'db dyn Db) -> Type<'db> {
+        IntersectionBuilder::new(db).add_negative(*self).build()
+    }
+
     pub const fn into_union_type(self) -> Option<UnionType<'db>> {
         match self {
             Type::Union(union_type) => Some(union_type),
diff --git a/crates/red_knot_python_semantic/src/types/builder.rs b/crates/red_knot_python_semantic/src/types/builder.rs
index 7a17fc1085..0a6e772831 100644
--- a/crates/red_knot_python_semantic/src/types/builder.rs
+++ b/crates/red_knot_python_semantic/src/types/builder.rs
@@ -173,14 +173,10 @@ impl<'db> IntersectionBuilder<'db> {
     pub(crate) fn add_negative(mut self, ty: Type<'db>) -> Self {
         // See comments above in `add_positive`; this is just the negated version.
         if let Type::Union(union) = ty {
-            union
-                .elements(self.db)
-                .iter()
-                .map(|elem| self.clone().add_negative(*elem))
-                .fold(IntersectionBuilder::empty(self.db), |mut builder, sub| {
-                    builder.intersections.extend(sub.intersections);
-                    builder
-                })
+            for elem in union.elements(self.db) {
+                self = self.add_negative(*elem);
+            }
+            self
         } else {
             for inner in &mut self.intersections {
                 inner.add_negative(self.db, ty);
@@ -667,6 +663,27 @@ mod tests {
         assert_eq!(ty, Type::IntLiteral(1));
     }
 
+    #[test]
+    fn build_negative_union_de_morgan() {
+        let db = setup_db();
+
+        let union = UnionBuilder::new(&db)
+            .add(Type::IntLiteral(1))
+            .add(Type::IntLiteral(2))
+            .build();
+        assert_eq!(union.display(&db).to_string(), "Literal[1, 2]");
+
+        let ty = IntersectionBuilder::new(&db).add_negative(union).build();
+
+        let expected = IntersectionBuilder::new(&db)
+            .add_negative(Type::IntLiteral(1))
+            .add_negative(Type::IntLiteral(2))
+            .build();
+
+        assert_eq!(ty.display(&db).to_string(), "~Literal[1] & ~Literal[2]");
+        assert_eq!(ty, expected);
+    }
+
     #[test]
     fn build_intersection_simplify_positive_type_and_positive_subtype() {
         let db = setup_db();
diff --git a/crates/red_knot_python_semantic/src/types/infer.rs b/crates/red_knot_python_semantic/src/types/infer.rs
index 7f932827f1..f3c6b10691 100644
--- a/crates/red_knot_python_semantic/src/types/infer.rs
+++ b/crates/red_knot_python_semantic/src/types/infer.rs
@@ -2415,7 +2415,6 @@ impl<'db> TypeInferenceBuilder<'db> {
                 } else {
                     None
                 };
-
                 let ty = bindings_ty(self.db, definitions, unbound_ty);
 
                 if ty.is_unbound() {
diff --git a/crates/red_knot_python_semantic/src/types/narrow.rs b/crates/red_knot_python_semantic/src/types/narrow.rs
index 93d9cb43bf..58a314523a 100644
--- a/crates/red_knot_python_semantic/src/types/narrow.rs
+++ b/crates/red_knot_python_semantic/src/types/narrow.rs
@@ -1,5 +1,5 @@
 use crate::semantic_index::ast_ids::HasScopedAstId;
-use crate::semantic_index::constraint::{Constraint, PatternConstraint};
+use crate::semantic_index::constraint::{Constraint, ConstraintNode, PatternConstraint};
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::{ScopeId, ScopedSymbolId, SymbolTable};
@@ -34,13 +34,19 @@ pub(crate) fn narrowing_constraint<'db>(
     constraint: Constraint<'db>,
     definition: Definition<'db>,
 ) -> Option<Type<'db>> {
-    match constraint {
-        Constraint::Expression(expression) => {
-            all_narrowing_constraints_for_expression(db, expression)
-                .get(&definition.symbol(db))
-                .copied()
+    match constraint.node {
+        ConstraintNode::Expression(expression) => {
+            if constraint.is_positive {
+                all_narrowing_constraints_for_expression(db, expression)
+                    .get(&definition.symbol(db))
+                    .copied()
+            } else {
+                all_negative_narrowing_constraints_for_expression(db, expression)
+                    .get(&definition.symbol(db))
+                    .copied()
+            }
         }
-        Constraint::Pattern(pattern) => all_narrowing_constraints_for_pattern(db, pattern)
+        ConstraintNode::Pattern(pattern) => all_narrowing_constraints_for_pattern(db, pattern)
             .get(&definition.symbol(db))
             .copied(),
     }
@@ -51,7 +57,7 @@ fn all_narrowing_constraints_for_pattern<'db>(
     db: &'db dyn Db,
     pattern: PatternConstraint<'db>,
 ) -> NarrowingConstraints<'db> {
-    NarrowingConstraintsBuilder::new(db, Constraint::Pattern(pattern)).finish()
+    NarrowingConstraintsBuilder::new(db, ConstraintNode::Pattern(pattern), true).finish()
 }
 
 #[salsa::tracked(return_ref)]
@@ -59,7 +65,15 @@ fn all_narrowing_constraints_for_expression<'db>(
     db: &'db dyn Db,
     expression: Expression<'db>,
 ) -> NarrowingConstraints<'db> {
-    NarrowingConstraintsBuilder::new(db, Constraint::Expression(expression)).finish()
+    NarrowingConstraintsBuilder::new(db, ConstraintNode::Expression(expression), true).finish()
+}
+
+#[salsa::tracked(return_ref)]
+fn all_negative_narrowing_constraints_for_expression<'db>(
+    db: &'db dyn Db,
+    expression: Expression<'db>,
+) -> NarrowingConstraints<'db> {
+    NarrowingConstraintsBuilder::new(db, ConstraintNode::Expression(expression), false).finish()
 }
 
 /// Generate a constraint from the *type* of the second argument of an `isinstance` call.
@@ -88,36 +102,39 @@ type NarrowingConstraints<'db> = FxHashMap<ScopedSymbolId, Type<'db>>;
 
 struct NarrowingConstraintsBuilder<'db> {
     db: &'db dyn Db,
-    constraint: Constraint<'db>,
+    constraint: ConstraintNode<'db>,
+    is_positive: bool,
     constraints: NarrowingConstraints<'db>,
 }
 
 impl<'db> NarrowingConstraintsBuilder<'db> {
-    fn new(db: &'db dyn Db, constraint: Constraint<'db>) -> Self {
+    fn new(db: &'db dyn Db, constraint: ConstraintNode<'db>, is_positive: bool) -> Self {
         Self {
             db,
             constraint,
+            is_positive,
             constraints: NarrowingConstraints::default(),
         }
     }
 
     fn finish(mut self) -> NarrowingConstraints<'db> {
         match self.constraint {
-            Constraint::Expression(expression) => self.evaluate_expression_constraint(expression),
-            Constraint::Pattern(pattern) => self.evaluate_pattern_constraint(pattern),
+            ConstraintNode::Expression(expression) => {
+                self.evaluate_expression_constraint(expression, self.is_positive);
+            }
+            ConstraintNode::Pattern(pattern) => self.evaluate_pattern_constraint(pattern),
         }
-
         self.constraints.shrink_to_fit();
         self.constraints
     }
 
-    fn evaluate_expression_constraint(&mut self, expression: Expression<'db>) {
+    fn evaluate_expression_constraint(&mut self, expression: Expression<'db>, is_positive: bool) {
         match expression.node_ref(self.db).node() {
             ast::Expr::Compare(expr_compare) => {
-                self.add_expr_compare(expr_compare, expression);
+                self.add_expr_compare(expr_compare, expression, is_positive);
             }
             ast::Expr::Call(expr_call) => {
-                self.add_expr_call(expr_call, expression);
+                self.add_expr_call(expr_call, expression, is_positive);
             }
             _ => {} // TODO other test expression kinds
         }
@@ -160,12 +177,17 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
 
     fn scope(&self) -> ScopeId<'db> {
         match self.constraint {
-            Constraint::Expression(expression) => expression.scope(self.db),
-            Constraint::Pattern(pattern) => pattern.scope(self.db),
+            ConstraintNode::Expression(expression) => expression.scope(self.db),
+            ConstraintNode::Pattern(pattern) => pattern.scope(self.db),
         }
     }
 
-    fn add_expr_compare(&mut self, expr_compare: &ast::ExprCompare, expression: Expression<'db>) {
+    fn add_expr_compare(
+        &mut self,
+        expr_compare: &ast::ExprCompare,
+        expression: Expression<'db>,
+        is_positive: bool,
+    ) {
         let ast::ExprCompare {
             range: _,
             left,
@@ -177,6 +199,13 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
             // we have no symbol to narrow down the type of.
             return;
         }
+        if !is_positive && comparators.len() > 1 {
+            // We can't negate a constraint made by a multi-comparator expression, since we can't
+            // know which comparison part is the one being negated.
+            // For example, the negation of  `x is 1 is y is 2`, would be `(x is not 1) or (y is not 1) or (y is not 2)`
+            // and that requires cross-symbol constraints, which we don't support yet.
+            return;
+        }
         let scope = self.scope();
         let inference = infer_expression_types(self.db, expression);
 
@@ -192,12 +221,13 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
             {
                 // SAFETY: we should always have a symbol for every Name node.
                 let symbol = self.symbols().symbol_id_by_name(id).unwrap();
-                let comp_ty = inference.expression_ty(right.scoped_ast_id(self.db, scope));
-                match op {
+                let rhs_ty = inference.expression_ty(right.scoped_ast_id(self.db, scope));
+
+                match if is_positive { *op } else { op.negate() } {
                     ast::CmpOp::IsNot => {
-                        if comp_ty.is_singleton() {
+                        if rhs_ty.is_singleton() {
                             let ty = IntersectionBuilder::new(self.db)
-                                .add_negative(comp_ty)
+                                .add_negative(rhs_ty)
                                 .build();
                             self.constraints.insert(symbol, ty);
                         } else {
@@ -205,12 +235,12 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
                         }
                     }
                     ast::CmpOp::Is => {
-                        self.constraints.insert(symbol, comp_ty);
+                        self.constraints.insert(symbol, rhs_ty);
                     }
                     ast::CmpOp::NotEq => {
-                        if comp_ty.is_single_valued(self.db) {
+                        if rhs_ty.is_single_valued(self.db) {
                             let ty = IntersectionBuilder::new(self.db)
-                                .add_negative(comp_ty)
+                                .add_negative(rhs_ty)
                                 .build();
                             self.constraints.insert(symbol, ty);
                         }
@@ -223,7 +253,12 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
         }
     }
 
-    fn add_expr_call(&mut self, expr_call: &ast::ExprCall, expression: Expression<'db>) {
+    fn add_expr_call(
+        &mut self,
+        expr_call: &ast::ExprCall,
+        expression: Expression<'db>,
+        is_positive: bool,
+    ) {
         let scope = self.scope();
         let inference = infer_expression_types(self.db, expression);
 
@@ -242,7 +277,11 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
 
                     // TODO: add support for PEP 604 union types on the right hand side:
                     // isinstance(x, str | (int | float))
-                    if let Some(constraint) = generate_isinstance_constraint(self.db, &rhs_type) {
+                    if let Some(mut constraint) = generate_isinstance_constraint(self.db, &rhs_type)
+                    {
+                        if !is_positive {
+                            constraint = constraint.negate(self.db);
+                        }
                         self.constraints.insert(symbol, constraint);
                     }
                 }
diff --git a/crates/ruff_python_ast/src/nodes.rs b/crates/ruff_python_ast/src/nodes.rs
index d2e584279c..a35868d6a0 100644
--- a/crates/ruff_python_ast/src/nodes.rs
+++ b/crates/ruff_python_ast/src/nodes.rs
@@ -3071,6 +3071,22 @@ impl CmpOp {
             CmpOp::NotIn => "not in",
         }
     }
+
+    #[must_use]
+    pub const fn negate(&self) -> Self {
+        match self {
+            CmpOp::Eq => CmpOp::NotEq,
+            CmpOp::NotEq => CmpOp::Eq,
+            CmpOp::Lt => CmpOp::GtE,
+            CmpOp::LtE => CmpOp::Gt,
+            CmpOp::Gt => CmpOp::LtE,
+            CmpOp::GtE => CmpOp::Lt,
+            CmpOp::Is => CmpOp::IsNot,
+            CmpOp::IsNot => CmpOp::Is,
+            CmpOp::In => CmpOp::NotIn,
+            CmpOp::NotIn => CmpOp::In,
+        }
+    }
 }
 
 impl fmt::Display for CmpOp {