diff --git a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not.md b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not.md
new file mode 100644
index 0000000000..0cf341496c
--- /dev/null
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals_not.md
@@ -0,0 +1,33 @@
+# Narrowing for `not` conditionals
+
+The `not` operator negates a constraint.
+
+## `not is None`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = None if bool_instance() else 1
+
+if not x is None:
+    reveal_type(x)  # revealed: Literal[1]
+else:
+    reveal_type(x)  # revealed: None
+
+reveal_type(x)  # revealed: None | Literal[1]
+```
+
+## `not isinstance`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x = 1 if bool_instance() else "a"
+
+if not isinstance(x, (int)):
+    reveal_type(x)  # revealed: Literal["a"]
+else:
+    reveal_type(x)  # revealed: Literal[1]
+```
diff --git a/crates/red_knot_python_semantic/src/types/narrow.rs b/crates/red_knot_python_semantic/src/types/narrow.rs
index 58a314523a..57798a9427 100644
--- a/crates/red_knot_python_semantic/src/types/narrow.rs
+++ b/crates/red_knot_python_semantic/src/types/narrow.rs
@@ -129,13 +129,30 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
     }
 
     fn evaluate_expression_constraint(&mut self, expression: Expression<'db>, is_positive: bool) {
-        match expression.node_ref(self.db).node() {
+        let expression_node = expression.node_ref(self.db).node();
+        self.evaluate_expression_node_constraint(expression_node, expression, is_positive);
+    }
+
+    fn evaluate_expression_node_constraint(
+        &mut self,
+        expression_node: &ruff_python_ast::Expr,
+        expression: Expression<'db>,
+        is_positive: bool,
+    ) {
+        match expression_node {
             ast::Expr::Compare(expr_compare) => {
                 self.add_expr_compare(expr_compare, expression, is_positive);
             }
             ast::Expr::Call(expr_call) => {
                 self.add_expr_call(expr_call, expression, is_positive);
             }
+            ast::Expr::UnaryOp(unary_op) if unary_op.op == ast::UnaryOp::Not => {
+                self.evaluate_expression_node_constraint(
+                    &unary_op.operand,
+                    expression,
+                    !is_positive,
+                );
+            }
             _ => {} // TODO other test expression kinds
         }
     }