diff --git a/Lib/test/test_tcl.py b/Lib/test/test_tcl.py
index 7062e7e..91fb28f 100644
--- a/Lib/test/test_tcl.py
+++ b/Lib/test/test_tcl.py
@@ -569,6 +569,7 @@
         for arg, res in testcases:
             self.assertEqual(split(arg), res)
 
+character_size = 4 if sys.maxunicode > 0xFFFF else 2
 
 class BigmemTclTest(unittest.TestCase):
 
@@ -578,10 +579,59 @@
     @test_support.cpython_only
     @unittest.skipUnless(INT_MAX < PY_SSIZE_T_MAX, "needs UINT_MAX < SIZE_MAX")
     @test_support.precisionbigmemtest(size=INT_MAX + 1, memuse=5, dry_run=False)
-    def test_huge_string(self, size):
+    def test_huge_string_call(self, size):
         value = ' ' * size
         self.assertRaises(OverflowError, self.interp.call, 'set', '_', value)
 
+    @test_support.cpython_only
+    @unittest.skipUnless(test_support.have_unicode, 'requires unicode support')
+    @unittest.skipUnless(INT_MAX < PY_SSIZE_T_MAX, "needs UINT_MAX < SIZE_MAX")
+    @test_support.precisionbigmemtest(size=INT_MAX + 1,
+                                      memuse=2*character_size + 2,
+                                      dry_run=False)
+    def test_huge_unicode_call(self, size):
+        value = unicode(' ') * size
+        self.assertRaises(OverflowError, self.interp.call, 'set', '_', value)
+
+
+    @test_support.cpython_only
+    @unittest.skipUnless(INT_MAX < PY_SSIZE_T_MAX, "needs UINT_MAX < SIZE_MAX")
+    @test_support.precisionbigmemtest(size=INT_MAX + 1, memuse=9, dry_run=False)
+    def test_huge_string_builtins(self, size):
+        value = '1' + ' ' * size
+        self.check_huge_string_builtins(value)
+
+    @test_support.cpython_only
+    @unittest.skipUnless(test_support.have_unicode, 'requires unicode support')
+    @unittest.skipUnless(INT_MAX < PY_SSIZE_T_MAX, "needs UINT_MAX < SIZE_MAX")
+    @test_support.precisionbigmemtest(size=INT_MAX + 1,
+                                      memuse=2*character_size + 7,
+                                      dry_run=False)
+    def test_huge_unicode_builtins(self, size):
+        value = unicode('1' + ' ' * size)
+        self.check_huge_string_builtins(value)
+
+    def check_huge_string_builtins(self, value):
+        self.assertRaises(OverflowError, self.interp.tk.getint, value)
+        self.assertRaises(OverflowError, self.interp.tk.getdouble, value)
+        self.assertRaises(OverflowError, self.interp.tk.getboolean, value)
+        self.assertRaises(OverflowError, self.interp.eval, value)
+        self.assertRaises(OverflowError, self.interp.evalfile, value)
+        self.assertRaises(OverflowError, self.interp.record, value)
+        self.assertRaises(OverflowError, self.interp.adderrorinfo, value)
+        self.assertRaises(OverflowError, self.interp.setvar, value, 'x', 'a')
+        self.assertRaises(OverflowError, self.interp.setvar, 'x', value, 'a')
+        self.assertRaises(OverflowError, self.interp.unsetvar, value)
+        self.assertRaises(OverflowError, self.interp.unsetvar, 'x', value)
+        self.assertRaises(OverflowError, self.interp.adderrorinfo, value)
+        self.assertRaises(OverflowError, self.interp.exprstring, value)
+        self.assertRaises(OverflowError, self.interp.exprlong, value)
+        self.assertRaises(OverflowError, self.interp.exprboolean, value)
+        self.assertRaises(OverflowError, self.interp.splitlist, value)
+        self.assertRaises(OverflowError, self.interp.split, value)
+        self.assertRaises(OverflowError, self.interp.createcommand, value, max)
+        self.assertRaises(OverflowError, self.interp.deletecommand, value)
+
 
 def setUpModule():
     if test_support.verbose:
diff --git a/Misc/NEWS b/Misc/NEWS
index 3538f88..1508fa7 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -18,6 +18,9 @@
 Library
 -------
 
+- Issue #21552: Fixed possible integer overflow of too long string lengths in
+  the tkinter module on 64-bit platforms.
+
 - Issue #14315: The zipfile module now ignores extra fields in the central
   directory that are too short to be parsed instead of letting a struct.unpack
   error bubble up as this "bad data" appears in many real world zip files in
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index 8d52b89..808d481 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -1021,6 +1021,16 @@
     0,                      /*tp_is_gc*/
 };
 
+#if PY_SIZE_MAX > INT_MAX
+#define CHECK_STRING_LENGTH(s) do {                                     \
+        if (s != NULL && strlen(s) >= INT_MAX) {                        \
+            PyErr_SetString(PyExc_OverflowError, "string is too long"); \
+            return NULL;                                                \
+        } } while(0)
+#else
+#define CHECK_STRING_LENGTH(s)
+#endif
+
 static Tcl_Obj*
 AsObj(PyObject *value)
 {
@@ -1486,6 +1496,7 @@
     if (!PyArg_ParseTuple(args, "s:eval", &script))
         return NULL;
 
+    CHECK_STRING_LENGTH(script);
     CHECK_TCL_APPARTMENT;
 
     ENTER_TCL
@@ -1532,6 +1543,7 @@
     if (!PyArg_ParseTuple(args, "s:evalfile", &fileName))
         return NULL;
 
+    CHECK_STRING_LENGTH(fileName);
     CHECK_TCL_APPARTMENT;
 
     ENTER_TCL
@@ -1553,9 +1565,10 @@
     PyObject *res = NULL;
     int err;
 
-    if (!PyArg_ParseTuple(args, "s", &script))
+    if (!PyArg_ParseTuple(args, "s:record", &script))
         return NULL;
 
+    CHECK_STRING_LENGTH(script);
     CHECK_TCL_APPARTMENT;
 
     ENTER_TCL
@@ -1576,6 +1589,7 @@
 
     if (!PyArg_ParseTuple(args, "s:adderrorinfo", &msg))
         return NULL;
+    CHECK_STRING_LENGTH(msg);
     CHECK_TCL_APPARTMENT;
 
     ENTER_TCL
@@ -1743,6 +1757,8 @@
         if (!PyArg_ParseTuple(args, "ssO:setvar",
                               &name1, &name2, &newValue))
             return NULL;
+        CHECK_STRING_LENGTH(name1);
+        CHECK_STRING_LENGTH(name2);
         /* XXX must hold tcl lock already??? */
         newval = AsObj(newValue);
         ENTER_TCL
@@ -1788,6 +1804,7 @@
                           varname_converter, &name1, &name2))
         return NULL;
 
+    CHECK_STRING_LENGTH(name2);
     ENTER_TCL
     tres = Tcl_GetVar2Ex(Tkapp_Interp(self), name1, name2, flags);
     ENTER_OVERLAP
@@ -1831,6 +1848,8 @@
     if (!PyArg_ParseTuple(args, "s|s:unsetvar", &name1, &name2))
         return NULL;
 
+    CHECK_STRING_LENGTH(name1);
+    CHECK_STRING_LENGTH(name2);
     ENTER_TCL
     code = Tcl_UnsetVar2(Tkapp_Interp(self), name1, name2, flags);
     ENTER_OVERLAP
@@ -1930,6 +1949,7 @@
     if (!PyArg_ParseTuple(args, "s:exprstring", &s))
         return NULL;
 
+    CHECK_STRING_LENGTH(s);
     CHECK_TCL_APPARTMENT;
 
     ENTER_TCL
@@ -1954,6 +1974,7 @@
     if (!PyArg_ParseTuple(args, "s:exprlong", &s))
         return NULL;
 
+    CHECK_STRING_LENGTH(s);
     CHECK_TCL_APPARTMENT;
 
     ENTER_TCL
@@ -1977,6 +1998,7 @@
 
     if (!PyArg_ParseTuple(args, "s:exprdouble", &s))
         return NULL;
+    CHECK_STRING_LENGTH(s);
     CHECK_TCL_APPARTMENT;
     PyFPE_START_PROTECT("Tkapp_ExprDouble", return 0)
     ENTER_TCL
@@ -2001,6 +2023,7 @@
 
     if (!PyArg_ParseTuple(args, "s:exprboolean", &s))
         return NULL;
+    CHECK_STRING_LENGTH(s);
     CHECK_TCL_APPARTMENT;
     ENTER_TCL
     retval = Tcl_ExprBoolean(Tkapp_Interp(self), s, &v);
@@ -2053,6 +2076,7 @@
     if (!PyArg_ParseTuple(args, "et:splitlist", "utf-8", &list))
         return NULL;
 
+    CHECK_STRING_LENGTH(list);
     if (Tcl_SplitList(Tkapp_Interp(self), list,
                       &argc, &argv) == TCL_ERROR)  {
         PyMem_Free(list);
@@ -2114,6 +2138,7 @@
 
     if (!PyArg_ParseTuple(args, "et:split", "utf-8", &list))
         return NULL;
+    CHECK_STRING_LENGTH(list);
     v = Split(list);
     PyMem_Free(list);
     return v;
@@ -2259,6 +2284,7 @@
 
     if (!PyArg_ParseTuple(args, "sO:createcommand", &cmdName, &func))
         return NULL;
+    CHECK_STRING_LENGTH(cmdName);
     if (!PyCallable_Check(func)) {
         PyErr_SetString(PyExc_TypeError, "command not callable");
         return NULL;
@@ -2322,6 +2348,7 @@
 
     if (!PyArg_ParseTuple(args, "s:deletecommand", &cmdName))
         return NULL;
+    CHECK_STRING_LENGTH(cmdName);
 
 #ifdef WITH_THREAD
     if (self->threaded && self->thread_id != Tcl_GetCurrentThread()) {
@@ -3130,6 +3157,10 @@
                           &interactive, &wantobjects, &wantTk,
                           &sync, &use))
         return NULL;
+    CHECK_STRING_LENGTH(screenName);
+    CHECK_STRING_LENGTH(baseName);
+    CHECK_STRING_LENGTH(className);
+    CHECK_STRING_LENGTH(use);
 
     return (PyObject *) Tkapp_New(screenName, baseName, className,
                                   interactive, wantobjects,     wantTk,