diff --git a/Lib/dbm/dumb.py b/Lib/dbm/dumb.py
index a080f4e865508b..77b46c5db9b9e0 100644
--- a/Lib/dbm/dumb.py
+++ b/Lib/dbm/dumb.py
@@ -144,6 +144,8 @@ def _verify_open(self):
     def __getitem__(self, key):
         if isinstance(key, str):
             key = key.encode('utf-8')
+        elif isinstance(key, bytearray):
+            key = bytes(key)
         self._verify_open()
         pos, siz = self._index[key]     # may raise KeyError
         with _io.open(self._datfile, 'rb') as f:
@@ -189,7 +191,9 @@ def __setitem__(self, key, val):
             raise error('The database is opened for reading only')
         if isinstance(key, str):
             key = key.encode('utf-8')
-        elif not isinstance(key, (bytes, bytearray)):
+        elif isinstance(key, bytearray):
+            key = bytes(key)
+        elif not isinstance(key, bytes):
             raise TypeError("keys must be bytes or strings")
         if isinstance(val, str):
             val = val.encode('utf-8')
@@ -226,6 +230,8 @@ def __delitem__(self, key):
             raise error('The database is opened for reading only')
         if isinstance(key, str):
             key = key.encode('utf-8')
+        elif isinstance(key, bytearray):
+            key = bytes(key)
         self._verify_open()
         self._modified = True
         # The blocks used by the associated value are lost.
@@ -249,6 +255,8 @@ def items(self):
     def __contains__(self, key):
         if isinstance(key, str):
             key = key.encode('utf-8')
+        elif isinstance(key, bytearray):
+            key = bytes(key)
         try:
             return key in self._index
         except TypeError:
diff --git a/Lib/test/test_dbm_dumb.py b/Lib/test/test_dbm_dumb.py
index d977a81876df65..c0a16d5e203419 100644
--- a/Lib/test/test_dbm_dumb.py
+++ b/Lib/test/test_dbm_dumb.py
@@ -41,6 +41,15 @@ def test_dumbdbm_creation(self):
                 f[key] = self._dict[key]
             self.read_helper(f)
 
+    def test_dumbdbm_bytearray_keys(self):
+        # gh-158217: bytearray keys must not raise TypeError:
+        with contextlib.closing(dumbdbm.open(_fname, 'c')) as f:
+            f[bytearray(b'key')] = b'value'
+            self.assertEqual(f[bytearray(b'key')], b'value')
+            self.assertIn(bytearray(b'key'), f)
+            del f[bytearray(b'key')]
+            self.assertNotIn(bytearray(b'key'), f)
+
     @unittest.skipUnless(hasattr(os, 'umask'), 'test needs os.umask()')
     @os_helper.skip_unless_working_chmod
     def test_dumbdbm_creation_mode(self):
diff --git a/Misc/NEWS.d/next/Library/2026-09-26-12-53-36.gh-issue-158217.AbCdEf.rst b/Misc/NEWS.d/next/Library/2026-09-26-12-53-36.gh-issue-158217.AbCdEf.rst
new file mode 100644
index 00000000000000..5845f23edd24c3
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-09-26-12-53-36.gh-issue-158217.AbCdEf.rst
@@ -0,0 +1,2 @@
+Fix :mod:`dbm.dumb` to accept ``bytearray`` keys, consistent with the
+other :mod:`dbm` backends. Patch by Tony Leung.