diff --git a/mypy/checker.py b/mypy/checker.py
index a7875cbd2226..7ad2c0bfb6aa 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -8427,12 +8427,12 @@ def conditional_types(
     proposed_type: Type
     remaining_type: Type
 
-    proper_type = get_proper_type(current_type)
+    p_current_type = get_proper_type(current_type)
     # factorize over union types: isinstance(A|B, C) -> yes = A_yes | B_yes
-    if isinstance(proper_type, UnionType):
+    if isinstance(p_current_type, UnionType):
         yes_items: list[Type] = []
         no_items: list[Type] = []
-        for union_item in proper_type.items:
+        for union_item in p_current_type.items:
             yes_type, no_type = conditional_types(
                 union_item,
                 proposed_type_ranges,
@@ -8458,7 +8458,7 @@ def conditional_types(
         items[i] = item
     proposed_type = get_proper_type(UnionType.make_union(items))
 
-    if isinstance(proper_type, AnyType):
+    if isinstance(p_current_type, AnyType):
         return proposed_type, current_type
     if isinstance(proposed_type, AnyType):
         # We don't really know much about the proposed type, so we shouldn't
@@ -8509,6 +8509,11 @@ def conditional_types(
         proposed_precise_type,
         consider_runtime_isinstance=consider_runtime_isinstance,
     )
+
+    # Avoid widening the type
+    if is_proper_subtype(p_current_type, proposed_type, ignore_promotions=True):
+        proposed_type = default if default is not None else current_type
+
     return proposed_type, remaining_type
 
 
diff --git a/test-data/unit/check-isinstance.test b/test-data/unit/check-isinstance.test
index d98c5424156d..88b87765e973 100644
--- a/test-data/unit/check-isinstance.test
+++ b/test-data/unit/check-isinstance.test
@@ -1583,6 +1583,61 @@ def f(x: Union[int, str], typ: type) -> None:
     reveal_type(x) # N: Revealed type is "builtins.int | builtins.str"
 [builtins fixtures/isinstancelist.pyi]
 
+[case testIsInstanceWithUnknownTypeMultipleNarrowing]
+# flags: --strict-equality --warn-unreachable --python-version 3.10
+from __future__ import annotations
+from typing import Iterable
+from typing_extensions import TypeAlias
+import types
+
+# Regression test for https://github.com/python/mypy/issues/21181
+# We don't have the same type context as with the real stubs, so sort of fake it
+_ClassInfoLike: TypeAlias = "type | tuple[_ClassInfoLike, ...]"
+
+class A: ...
+class B(A): ...
+
+def fake_type_context(ts: list[type[A]]) -> _ClassInfoLike:
+    return tuple(ts)  # E: Too many arguments for "tuple"
+
+
+def f1(x: A | None) -> None:
+    if x is not None:
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+        if isinstance(x, object):
+            reveal_type(x)  # N: Revealed type is "__main__.A"
+
+
+def f2(x: A | None, ts: list[type[A]]) -> None:
+    if x is not None:
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+        if isinstance(x, fake_type_context(ts)):
+            reveal_type(x)  # N: Revealed type is "__main__.A"
+
+
+def f3(x: A | None, t: type | type[A]) -> None:
+    if x is not None:
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+        if isinstance(x, t):
+            reveal_type(x)  # N: Revealed type is "__main__.A"
+
+
+def f4(x: A | None, t: type) -> None:
+    if x is not None:
+        reveal_type(x)  # N: Revealed type is "__main__.A"
+        if isinstance(x, t):
+            reveal_type(x)  # N: Revealed type is "__main__.A"
+
+
+def f5(x: object | None, ta: type[A], tb: type[B]) -> None:
+    if x is not None:
+        reveal_type(x)  # N: Revealed type is "builtins.object"
+        if isinstance(x, ta):
+            reveal_type(x)  # N: Revealed type is "__main__.A"
+            if isinstance(x, tb):
+                reveal_type(x)  # N: Revealed type is "__main__.B"
+[builtins fixtures/isinstancelist.pyi]
+
 [case testIsInstanceWithBoundedType]
 # flags: --warn-unreachable
 from typing import Union, Type