diff -r d35fc6e58a70 Lib/tempfile.py
--- a/Lib/tempfile.py	Sun Dec 04 22:59:09 2016 +0100
+++ b/Lib/tempfile.py	Tue Dec 06 21:25:09 2016 -0800
@@ -447,7 +447,10 @@
 
         # Need to ensure the file is deleted on __del__
         def __del__(self):
-            self.close()
+            if not self.close_called and self.file is not None:
+                _warnings.warn('unclosed file %r' % self, ResourceWarning,
+                               stacklevel=2, source=self)
+                self.close()
 
     else:
         def close(self):
diff -r d35fc6e58a70 Lib/test/test_tempfile.py
--- a/Lib/test/test_tempfile.py	Sun Dec 04 22:59:09 2016 +0100
+++ b/Lib/test/test_tempfile.py	Tue Dec 06 21:25:09 2016 -0800
@@ -957,6 +957,14 @@
             tempfile.NamedTemporaryFile(mode=2, dir=dir)
         self.assertEqual(os.listdir(dir), [])
 
+    def test_resource_warning_on_destructor(self):
+        file = tempfile.NamedTemporaryFile()
+        with warnings.catch_warnings(record=True) as w:
+            del file
+        self.assertEqual(len(w), 1)
+        self.assertEqual(w[-1].category, ResourceWarning)
+        self.assertIn('unclosed file', str(w[-1].message))
+
     # How to test the mode and bufsize parameters?
 
 class TestSpooledTemporaryFile(BaseTestCase):