diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
@@ -1205,6 +1205,18 @@ always available.
 
    .. versionadded:: 3.12
 
+.. function:: _is_interned(string)
+
+   Return :const:`True` if the given string is "interned", :const:`False`
+   otherwise.
+
+   .. versionadded:: 3.13
+
+   .. impl-detail::
+
+      It is not guaranteed to exist in all implementations of Python.
+
+
 .. data:: last_type
           last_value
           last_traceback

@@ -297,6 +297,13 @@ sqlite3
   object is not :meth:`closed <sqlite3.Connection.close>` explicitly.
   (Contributed by Erlend E. Aasland in :gh:`105539`.)
 
+sys
+---
+
+* Add the :func:`sys._is_interned` function to test if the string was interned.
+  This function is not guaranteed to exist in all implementations of Python.
+  (Contributed by Serhiy Storchaka in :gh:`78573`.)
+
 tkinter
 -------
 

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
@@ -691,11 +691,23 @@ def test_43581(self):
         self.assertEqual(sys.__stdout__.encoding, sys.__stderr__.encoding)
 
     def test_intern(self):
+        has_is_interned = (test.support.check_impl_detail(cpython=True)
+                           or hasattr(sys, '_is_interned'))
         self.assertRaises(TypeError, sys.intern)
+        self.assertRaises(TypeError, sys.intern, b'abc')
+        if has_is_interned:
+            self.assertRaises(TypeError, sys._is_interned)
+            self.assertRaises(TypeError, sys._is_interned, b'abc')
         s = "never interned before" + str(random.randrange(0, 10**9))
         self.assertTrue(sys.intern(s) is s)
+        if has_is_interned:
+            self.assertIs(sys._is_interned(s), True)
         s2 = s.swapcase().swapcase()
+        if has_is_interned:
+            self.assertIs(sys._is_interned(s2), False)
         self.assertTrue(sys.intern(s2) is s)
+        if has_is_interned:
+            self.assertIs(sys._is_interned(s2), False)
 
         # Subclasses of string can't be interned, because they
         # provide too much opportunity for insane things to happen.
@@ -707,6 +719,8 @@ def __hash__(self):
                 return 123
 
         self.assertRaises(TypeError, sys.intern, S("abc"))
+        if has_is_interned:
+            self.assertIs(sys._is_interned(S("abc")), False)
 
     @requires_subinterpreters
     def test_subinterp_intern_dynamically_allocated(self):

diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-08-13-13-25-15.bpo-34392.9kIlMF.rst b/Misc/NEWS.d/next/Core and Builtins/2018-08-13-13-25-15.bpo-34392.9kIlMF.rst
@@ -0,0 +1 @@
+Added :func:`sys._is_interned`.
diff --git a/Python/clinic/sysmodule.c.h b/Python/clinic/sysmodule.c.h
@@ -989,6 +989,23 @@ sys_intern_impl(PyObject *module, PyObject *s)
 }
 
 
+/*[clinic input]
+sys._is_interned -> bool
+
+  string: unicode
+  /
+
+Return True if the given string is "interned".
+[clinic start generated code]*/
+
+static int
+sys__is_interned_impl(PyObject *module, PyObject *string)
+/*[clinic end generated code: output=c3678267b4e9d7ed input=039843e17883b606]*/
+{
+    return PyUnicode_CHECK_INTERNED(string);
+}
+
+
 /*
  * Cached interned string objects used for calling the profile and
  * trace functions.
@@ -2462,6 +2479,7 @@ static PyMethodDef sys_methods[] = {
     SYS_GETWINDOWSVERSION_METHODDEF
     SYS__ENABLELEGACYWINDOWSFSENCODING_METHODDEF
     SYS_INTERN_METHODDEF
+    SYS__IS_INTERNED_METHODDEF
     SYS_IS_FINALIZING_METHODDEF
     SYS_MDEBUG_METHODDEF
     SYS_SETSWITCHINTERVAL_METHODDEF