diff --git a/crates/ruff_linter/resources/test/fixtures/ruff/suppressions.py b/crates/ruff_linter/resources/test/fixtures/ruff/suppressions.py
index 2eabfcde89..2a4b06898a 100644
--- a/crates/ruff_linter/resources/test/fixtures/ruff/suppressions.py
+++ b/crates/ruff_linter/resources/test/fixtures/ruff/suppressions.py
@@ -95,3 +95,10 @@ def f():
     value = 0
     # ruff: enable[F841, RQW320]
     # ruff: enable[YF829]
+
+
+def f():
+    # External rule codes should be ignored
+    # ruff: disable[TK421]
+    print("hello")
+    # ruff: enable[TK421]
diff --git a/crates/ruff_linter/src/rules/ruff/mod.rs b/crates/ruff_linter/src/rules/ruff/mod.rs
index ad18e24d59..9bc6363731 100644
--- a/crates/ruff_linter/src/rules/ruff/mod.rs
+++ b/crates/ruff_linter/src/rules/ruff/mod.rs
@@ -316,7 +316,8 @@ mod tests {
                 Rule::InvalidRuleCode,
                 Rule::InvalidSuppressionComment,
                 Rule::UnmatchedSuppressionComment,
-            ]),
+            ])
+            .with_external_rules(&["TK421"]),
             &settings::LinterSettings::for_rules(vec![
                 Rule::UnusedVariable,
                 Rule::AmbiguousVariableName,
@@ -325,6 +326,7 @@ mod tests {
                 Rule::InvalidSuppressionComment,
                 Rule::UnmatchedSuppressionComment,
             ])
+            .with_external_rules(&["TK421"])
             .with_preview_mode(),
         );
         Ok(())
diff --git a/crates/ruff_linter/src/settings/mod.rs b/crates/ruff_linter/src/settings/mod.rs
index 5d5e35aa8d..2f22f91b6b 100644
--- a/crates/ruff_linter/src/settings/mod.rs
+++ b/crates/ruff_linter/src/settings/mod.rs
@@ -471,6 +471,13 @@ impl LinterSettings {
         self
     }
 
+    #[must_use]
+    pub fn with_external_rules(mut self, rules: &[&str]) -> Self {
+        self.external
+            .extend(rules.iter().map(std::string::ToString::to_string));
+        self
+    }
+
     /// Resolve the [`TargetVersion`] to use for linting.
     ///
     /// This method respects the per-file version overrides in