diff --git a/mypy/checker.py b/mypy/checker.py
index 2cb3d69b2d77..f51a1e81572d 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -6831,6 +6831,16 @@ def comparison_type_narrowing_helper(self, node: ComparisonExpr) -> tuple[TypeMa
                             # We can't do negative narrowing, since e.g. the container could
                             # just be empty.
                             else_map = {}
+                            # The `in` operator uses __eq__, not isinstance. Knowing that
+                            # value == some_container_element does not imply value has the
+                            # runtime type of that element. Discard any narrowing that would
+                            # widen item_type to a non-subtype (e.g. tuple[int,int,int] must
+                            # not be widened to a subclass Size just because they compare equal).
+                            left_expr = collapse_walrus(operands[left_index])
+                            if left_expr in if_map and not is_proper_subtype(
+                                if_map[left_expr], item_type, ignore_promotions=True
+                            ):
+                                del if_map[left_expr]
 
                 if right_index in narrowable_operand_index_to_hash:
                     # E.g. narrows the right operand in `if "key" in typed_dict`
diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 7bab7baa6ceb..ec068d5210dc 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -4281,3 +4281,17 @@ def func(y: H) -> H:
     else:
         return y
 [builtins fixtures/primitives.pyi]
+
+[case testValueInContainerNoWideningToNonSubtype]
+# Regression test for https://github.com/python/mypy/issues/21512
+# Narrowing x in `if x in container` must not widen x to the container item
+# type when that item type is not a subtype of x's declared type.
+class Size(tuple[int, ...]):
+    def numel(self) -> int: ...
+
+sizes: list[Size]
+value = (1, 2, 3)
+if value in sizes:
+    reveal_type(value)  # N: Revealed type is "tuple[builtins.int, builtins.int, builtins.int]"
+    value.numel()  # E: "tuple[int, int, int]" has no attribute "numel"
+[builtins fixtures/primitives.pyi]