diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index a611142aed..8487a0b1d9 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -709,27 +709,63 @@ def test_hash(self):
         self.assertEqual(hash(int | str), hash(str | int))
         self.assertEqual(hash(int | str), hash(typing.Union[int, str]))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
-    def test_instancecheck(self):
-        x = int | str
-        self.assertIsInstance(1, x)
-        self.assertIsInstance(True, x)
-        self.assertIsInstance('a', x)
-        self.assertNotIsInstance(None, x)
-        self.assertTrue(issubclass(int, x))
-        self.assertTrue(issubclass(bool, x))
-        self.assertTrue(issubclass(str, x))
-        self.assertFalse(issubclass(type(None), x))
-        x = int | None
-        self.assertIsInstance(None, x)
-        self.assertTrue(issubclass(type(None), x))
-        x = int | collections.abc.Mapping
-        self.assertIsInstance({}, x)
-        self.assertTrue(issubclass(dict, x))
+    def test_instancecheck_and_subclasscheck(self):
+        for x in (int | str, typing.Union[int, str]):
+            with self.subTest(x=x):
+                self.assertIsInstance(1, x)
+                self.assertIsInstance(True, x)
+                self.assertIsInstance('a', x)
+                self.assertNotIsInstance(None, x)
+                self.assertTrue(issubclass(int, x))
+                self.assertTrue(issubclass(bool, x))
+                self.assertTrue(issubclass(str, x))
+                self.assertFalse(issubclass(type(None), x))
+
+        for x in (int | None, typing.Union[int, None]):
+            with self.subTest(x=x):
+                self.assertIsInstance(None, x)
+                self.assertTrue(issubclass(type(None), x))
+
+        for x in (
+            int | collections.abc.Mapping,
+            typing.Union[int, collections.abc.Mapping],
+        ):
+            with self.subTest(x=x):
+                self.assertIsInstance({}, x)
+                self.assertNotIsInstance((), x)
+                self.assertTrue(issubclass(dict, x))
+                self.assertFalse(issubclass(list, x))
+
+    def test_instancecheck_and_subclasscheck_order(self):
+        T = typing.TypeVar('T')
+
+        will_resolve = (
+            int | T,
+            typing.Union[int, T],
+        )
+        for x in will_resolve:
+            with self.subTest(x=x):
+                self.assertIsInstance(1, x)
+                self.assertTrue(issubclass(int, x))
+
+        wont_resolve = (
+            T | int,
+            typing.Union[T, int],
+        )
+        for x in wont_resolve:
+            with self.subTest(x=x):
+                with self.assertRaises(TypeError):
+                    issubclass(int, x)
+                with self.assertRaises(TypeError):
+                    isinstance(1, x)
+
+        for x in (*will_resolve, *wont_resolve):
+            with self.subTest(x=x):
+                with self.assertRaises(TypeError):
+                    issubclass(object, x)
+                with self.assertRaises(TypeError):
+                    isinstance(object(), x)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_bad_instancecheck(self):
         class BadMeta(type):
             def __instancecheck__(cls, inst):
@@ -738,8 +774,6 @@ def __instancecheck__(cls, inst):
         self.assertTrue(isinstance(1, x))
         self.assertRaises(ZeroDivisionError, isinstance, [], x)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_bad_subclasscheck(self):
         class BadMeta(type):
             def __subclasscheck__(cls, sub):
diff --git a/vm/src/builtins/union.rs b/vm/src/builtins/union.rs
index d43fbc10b6..461c1af37e 100644
--- a/vm/src/builtins/union.rs
+++ b/vm/src/builtins/union.rs
@@ -90,15 +90,33 @@ impl PyUnion {
     }
 
     #[pymethod(magic)]
-    fn instancecheck(_zelf: PyRef<Self>, _obj: PyObjectRef, vm: &VirtualMachine) -> PyResult {
-        Err(vm
-            .new_type_error("isinstance() argument 2 cannot be a parameterized generic".to_owned()))
+    fn instancecheck(zelf: PyRef<Self>, obj: PyObjectRef, vm: &VirtualMachine) -> PyResult<bool> {
+        if zelf
+            .args
+            .iter()
+            .any(|x| x.class().is(vm.ctx.types.generic_alias_type))
+        {
+            Err(vm.new_type_error(
+                "isinstance() argument 2 cannot be a parameterized generic".to_owned(),
+            ))
+        } else {
+            obj.is_instance(zelf.args().as_object(), vm)
+        }
     }
 
     #[pymethod(magic)]
-    fn subclasscheck(_zelf: PyRef<Self>, _obj: PyObjectRef, vm: &VirtualMachine) -> PyResult {
-        Err(vm
-            .new_type_error("issubclass() argument 2 cannot be a parameterized generic".to_owned()))
+    fn subclasscheck(zelf: PyRef<Self>, obj: PyObjectRef, vm: &VirtualMachine) -> PyResult<bool> {
+        if zelf
+            .args
+            .iter()
+            .any(|x| x.class().is(vm.ctx.types.generic_alias_type))
+        {
+            Err(vm.new_type_error(
+                "issubclass() argument 2 cannot be a parameterized generic".to_owned(),
+            ))
+        } else {
+            obj.is_subclass(zelf.args().as_object(), vm)
+        }
     }
 
     #[pymethod(name = "__ror__")]
diff --git a/vm/src/protocol/object.rs b/vm/src/protocol/object.rs
index 79e252c4e2..ca7aa1455e 100644
--- a/vm/src/protocol/object.rs
+++ b/vm/src/protocol/object.rs
@@ -398,7 +398,7 @@ impl PyObject {
             })
             .and(self.check_cls(cls, vm, || {
                 format!(
-                    "issubclass() arg 2 must be a class or tuple of classes, not {}",
+                    "issubclass() arg 2 must be a class, a tuple of classes, or a union, not {}",
                     cls.class()
                 )
             }))