diff --git a/Doc/library/filecmp.rst b/Doc/library/filecmp.rst
--- a/Doc/library/filecmp.rst
+++ b/Doc/library/filecmp.rst
@@ -152,5 +152,6 @@
    .. attribute:: subdirs
 
       A dictionary mapping names in :attr:`common_dirs` to :class:`dircmp`
-      objects.
+      instances (or to instances of the object's class if :class:`dircmp`
+      was subclassed).
 
diff --git a/Lib/filecmp.py b/Lib/filecmp.py
--- a/Lib/filecmp.py
+++ b/Lib/filecmp.py
@@ -182,7 +182,7 @@
         for x in self.common_dirs:
             a_x = os.path.join(self.left, x)
             b_x = os.path.join(self.right, x)
-            self.subdirs[x]  = dircmp(a_x, b_x, self.ignore, self.hide)
+            self.subdirs[x]  = self.__class__(a_x, b_x, self.ignore, self.hide)
 
     def phase4_closure(self): # Recursively call phase4() on subdirectories
         self.phase4()
diff --git a/Lib/test/test_filecmp.py b/Lib/test/test_filecmp.py
--- a/Lib/test/test_filecmp.py
+++ b/Lib/test/test_filecmp.py
@@ -51,6 +51,8 @@
         for dir in [self.dir, self.dir_same, self.dir_diff]:
             shutil.rmtree(dir, True)
             os.mkdir(dir)
+            subdir_path = os.path.join(dir, 'subdir')
+            os.mkdir(subdir_path)
             if self.caseinsensitive and dir is self.dir_same:
                 fn = 'FiLe'     # Verify case-insensitive comparison
             else:
@@ -95,24 +97,31 @@
                     (['file'], ['file2'], []),
                     "Comparing mismatched directories fails")
 
+    def _assert_lists(self, actual, expected):
+        """Assert that two lists are equal, up to ordering."""
+        self.assertEqual(sorted(actual), sorted(expected))
 
     def test_dircmp(self):
         # Check attributes for comparison of two identical directories
         d = filecmp.dircmp(self.dir, self.dir_same)
         if self.caseinsensitive:
-            self.assertEqual([d.left_list, d.right_list],[['file'], ['FiLe']])
+            self._assert_lists(d.left_list, ['file', 'subdir'])
+            self._assert_lists(d.right_list, ['file', 'subdir'])
         else:
-            self.assertEqual([d.left_list, d.right_list],[['file'], ['file']])
-        self.assertEqual(d.common, ['file'])
+            self._assert_lists(d.left_list, ['file', 'subdir'])
+            self._assert_lists(d.right_list, ['file', 'subdir'])
+        self._assert_lists(d.common, ['file', 'subdir'])
+        self.assertEqual(d.common_dirs, ['subdir'])
         self.assertTrue(d.left_only == d.right_only == [])
         self.assertEqual(d.same_files, ['file'])
         self.assertEqual(d.diff_files, [])
 
         # Check attributes for comparison of two different directories
         d = filecmp.dircmp(self.dir, self.dir_diff)
-        self.assertEqual(d.left_list, ['file'])
-        self.assertTrue(d.right_list == ['file', 'file2'])
-        self.assertEqual(d.common, ['file'])
+        self._assert_lists(d.left_list, ['file', 'subdir'])
+        self._assert_lists(d.right_list, ['file', 'file2', 'subdir'])
+        self._assert_lists(d.common, ['file', 'subdir'])
+        self.assertEqual(d.common_dirs, ['subdir'])
         self.assertEqual(d.left_only, [])
         self.assertEqual(d.right_only, ['file2'])
         self.assertEqual(d.same_files, ['file'])
@@ -126,6 +135,16 @@
         self.assertEqual(d.same_files, ['file'])
         self.assertEqual(d.diff_files, ['file2'])
 
+    def test_dircmp_subdirs_type(self):
+        """Check that dircmp.subdirs respects subclassing."""
+        class MyDirCmp(filecmp.dircmp):
+            pass
+        d = MyDirCmp(self.dir, self.dir_diff)
+        sub_dirs = d.subdirs
+        self.assertEqual(list(sub_dirs.keys()), ['subdir'])
+        sub_dcmp = sub_dirs['subdir']
+        self.assertEqual(type(sub_dcmp), MyDirCmp)
+
 
 def test_main():
     support.run_unittest(FileCompareTestCase, DirCompareTestCase)
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -55,6 +55,9 @@
 Library
 -------
 
+- Issue #15450: Ensure that filecmp's dircmp.subdirs behaves as expected
+  when subclassing dircmp.  Patch by Chris Jerdonek.
+
 - Issue #6056: Make multiprocessing use setblocking(True) on the
   sockets it uses.  Original patch by J Derek Wilson.