diff --git a/crates/pep508-rs/src/lib.rs b/crates/pep508-rs/src/lib.rs
index f4d8d64a6..a9e072b27 100644
--- a/crates/pep508-rs/src/lib.rs
+++ b/crates/pep508-rs/src/lib.rs
@@ -1710,4 +1710,10 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    fn no_space_after_operator() {
+        let requirement = Requirement::from_str("pytest;'4.0'>=python_version").unwrap();
+        assert_eq!(requirement.to_string(), "pytest ; '4.0' >= python_version");
+    }
 }
diff --git a/crates/pep508-rs/src/marker.rs b/crates/pep508-rs/src/marker.rs
index 0fce8e3e6..fb723daa0 100644
--- a/crates/pep508-rs/src/marker.rs
+++ b/crates/pep508-rs/src/marker.rs
@@ -1150,9 +1150,15 @@ impl Display for MarkerTree {
 /// version_cmp   = wsp* <'<=' | '<' | '!=' | '==' | '>=' | '>' | '~=' | '==='>
 /// marker_op     = version_cmp | (wsp* 'in') | (wsp* 'not' wsp+ 'in')
 /// ```
+/// The `wsp*` has already been consumed by the caller.
 fn parse_marker_operator(cursor: &mut Cursor) -> Result<MarkerOperator, Pep508Error> {
-    let (start, len) =
-        cursor.take_while(|char| !char.is_whitespace() && char != '\'' && char != '"');
+    let (start, len) = if cursor.peek_char().is_some_and(|c| c.is_alphabetic()) {
+        // "in" or "not"
+        cursor.take_while(|char| !char.is_whitespace() && char != '\'' && char != '"')
+    } else {
+        // A mathematical operator
+        cursor.take_while(|char| matches!(char, '<' | '=' | '>' | '~' | '!'))
+    };
     let operator = cursor.slice(start, len);
     if operator == "not" {
         // 'not' wsp+ 'in'