diff --git a/Include/internal/pycore_unionobject.h b/Include/internal/pycore_unionobject.h
index fa8ba6ed944..4d82b6fbeae 100644
--- a/Include/internal/pycore_unionobject.h
+++ b/Include/internal/pycore_unionobject.h
@@ -10,6 +10,7 @@ extern "C" {
 
 PyAPI_FUNC(PyObject *) _Py_Union(PyObject *args);
 PyAPI_DATA(PyTypeObject) _Py_UnionType;
+PyAPI_FUNC(PyObject *) _Py_union_type_or(PyObject* self, PyObject* param);
 
 #ifdef __cplusplus
 }
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index 75c5eee42dc..3058a02d6ee 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -713,6 +713,28 @@ class TypesTests(unittest.TestCase):
         assert repr(int | None) == "int | None"
         assert repr(int | typing.GenericAlias(list, int)) == "int | list[int]"
 
+    def test_or_type_operator_with_genericalias(self):
+        a = list[int]
+        b = list[str]
+        c = dict[float, str]
+        # equivalence with typing.Union
+        self.assertEqual(a | b | c, typing.Union[a, b, c])
+        # de-duplicate
+        self.assertEqual(a | c | b | b | a | c, a | b | c)
+        # order shouldn't matter
+        self.assertEqual(a | b, b | a)
+        self.assertEqual(repr(a | b | c),
+                         "list[int] | list[str] | dict[float, str]")
+
+        class BadType(type):
+            def __eq__(self, other):
+                return 1 / 0
+
+        bt = BadType('bt', (), {})
+        # Comparison should fail and errors should propagate out for bad types.
+        with self.assertRaises(ZeroDivisionError):
+            list[int] | list[bt]
+
     def test_ellipsis_type(self):
         self.assertIsInstance(Ellipsis, types.EllipsisType)
 
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-11-01-23-34-56.bpo-42233.zOSzja.rst b/Misc/NEWS.d/next/Core and Builtins/2020-11-01-23-34-56.bpo-42233.zOSzja.rst
new file mode 100644
index 00000000000..499bb324fb9
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-11-01-23-34-56.bpo-42233.zOSzja.rst	
@@ -0,0 +1,5 @@
+Allow ``GenericAlias`` objects to use :ref:`union type expressions <types-union>`.
+This allows expressions like ``list[int] | dict[float, str]`` where previously a
+``TypeError`` would have been thrown.  This also fixes union type expressions 
+not de-duplicating ``GenericAlias`` objects.  (Contributed by Ken Jin in 
+:issue:`42233`.)
diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index 6508c69cbf7..28ea487a44f 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -2,6 +2,7 @@
 
 #include "Python.h"
 #include "pycore_object.h"
+#include "pycore_unionobject.h"   // _Py_union_as_number
 #include "structmember.h"         // PyMemberDef
 
 typedef struct {
@@ -573,6 +574,10 @@ ga_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     return Py_GenericAlias(origin, arguments);
 }
 
+static PyNumberMethods ga_as_number = {
+        .nb_or = (binaryfunc)_Py_union_type_or, // Add __or__ function
+};
+
 // TODO:
 // - argument clinic?
 // - __doc__?
@@ -586,6 +591,7 @@ PyTypeObject Py_GenericAliasType = {
     .tp_basicsize = sizeof(gaobject),
     .tp_dealloc = ga_dealloc,
     .tp_repr = ga_repr,
+    .tp_as_number = &ga_as_number,  // allow X | Y of GenericAlias objs
     .tp_as_mapping = &ga_as_mapping,
     .tp_hash = ga_hash,
     .tp_call = ga_call,
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 3822b8cf813..55bf9b3f389 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -6,7 +6,7 @@
 #include "pycore_object.h"
 #include "pycore_pyerrors.h"
 #include "pycore_pystate.h"       // _PyThreadState_GET()
-#include "pycore_unionobject.h"   // _Py_Union()
+#include "pycore_unionobject.h"   // _Py_Union(), _Py_union_type_or
 #include "frameobject.h"
 #include "structmember.h"         // PyMemberDef
 
@@ -3789,19 +3789,9 @@ type_is_gc(PyTypeObject *type)
     return type->tp_flags & Py_TPFLAGS_HEAPTYPE;
 }
 
-static PyObject *
-type_or(PyTypeObject* self, PyObject* param) {
-    PyObject *tuple = PyTuple_Pack(2, self, param);
-    if (tuple == NULL) {
-        return NULL;
-    }
-    PyObject *new_union = _Py_Union(tuple);
-    Py_DECREF(tuple);
-    return new_union;
-}
 
 static PyNumberMethods type_as_number = {
-        .nb_or = (binaryfunc)type_or, // Add __or__ function
+        .nb_or = _Py_union_type_or, // Add __or__ function
 };
 
 PyTypeObject PyType_Type = {
diff --git a/Objects/unionobject.c b/Objects/unionobject.c
index 1b7f8ab51a4..2308bfc9f2a 100644
--- a/Objects/unionobject.c
+++ b/Objects/unionobject.c
@@ -237,9 +237,19 @@ dedup_and_flatten_args(PyObject* args)
         PyObject* i_element = PyTuple_GET_ITEM(args, i);
         for (Py_ssize_t j = i + 1; j < arg_length; j++) {
             PyObject* j_element = PyTuple_GET_ITEM(args, j);
-            if (i_element == j_element) {
-                is_duplicate = 1;
+            int is_ga = Py_TYPE(i_element) == &Py_GenericAliasType &&
+                        Py_TYPE(j_element) == &Py_GenericAliasType;
+            // RichCompare to also deduplicate GenericAlias types (slower)
+            is_duplicate = is_ga ? PyObject_RichCompareBool(i_element, j_element, Py_EQ)
+                : i_element == j_element;
+            // Should only happen if RichCompare fails
+            if (is_duplicate < 0) {
+                Py_DECREF(args);
+                Py_DECREF(new_args);
+                return NULL;
             }
+            if (is_duplicate)
+                break;
         }
         if (!is_duplicate) {
             Py_INCREF(i_element);
@@ -290,8 +300,8 @@ is_unionable(PyObject *obj)
         type == &_Py_UnionType);
 }
 
-static PyObject *
-type_or(PyTypeObject* self, PyObject* param)
+PyObject *
+_Py_union_type_or(PyObject* self, PyObject* param)
 {
     PyObject *tuple = PyTuple_Pack(2, self, param);
     if (tuple == NULL) {
@@ -404,7 +414,7 @@ static PyMethodDef union_methods[] = {
         {0}};
 
 static PyNumberMethods union_as_number = {
-        .nb_or = (binaryfunc)type_or, // Add __or__ function
+        .nb_or = _Py_union_type_or, // Add __or__ function
 };
 
 PyTypeObject _Py_UnionType = {