diff --git a/Lib/test/test_tcl.py b/Lib/test/test_tcl.py
index 3183ea850f7..6005349eaac 100644
--- a/Lib/test/test_tcl.py
+++ b/Lib/test/test_tcl.py
@@ -1,4 +1,5 @@
 import unittest
+import locale
 import re
 import subprocess
 import sys
@@ -58,6 +59,10 @@ class TclTest(unittest.TestCase):
         tcl = self.interp
         self.assertEqual(tcl.eval('set a "a\\0b"'), 'a\x00b')
 
+    def test_eval_surrogates_in_result(self):
+        tcl = self.interp
+        self.assertIn(tcl.eval(r'set a "<\ud83d\udcbb>"'), '<\U0001f4bb>')
+
     def testEvalException(self):
         tcl = self.interp
         self.assertRaises(TclError,tcl.eval,'set a')
@@ -190,29 +195,48 @@ class TclTest(unittest.TestCase):
 
     def testEvalFile(self):
         tcl = self.interp
-        with open(support.TESTFN, 'w') as f:
-            self.addCleanup(support.unlink, support.TESTFN)
+        filename = support.TESTFN
+        self.addCleanup(support.unlink, filename)
+        with open(filename, 'w') as f:
             f.write("""set a 1
             set b 2
             set c [ expr $a + $b ]
             """)
-        tcl.evalfile(support.TESTFN)
+        tcl.evalfile(filename)
         self.assertEqual(tcl.eval('set a'),'1')
         self.assertEqual(tcl.eval('set b'),'2')
         self.assertEqual(tcl.eval('set c'),'3')
 
     def test_evalfile_null_in_result(self):
         tcl = self.interp
-        with open(support.TESTFN, 'w') as f:
-            self.addCleanup(support.unlink, support.TESTFN)
+        filename = support.TESTFN
+        self.addCleanup(support.unlink, filename)
+        with open(filename, 'w') as f:
             f.write("""
             set a "a\0b"
             set b "a\\0b"
             """)
-        tcl.evalfile(support.TESTFN)
+        tcl.evalfile(filename)
         self.assertEqual(tcl.eval('set a'), 'a\x00b')
         self.assertEqual(tcl.eval('set b'), 'a\x00b')
 
+    def test_evalfile_surrogates_in_result(self):
+        tcl = self.interp
+        encoding = tcl.call('encoding', 'system')
+        self.addCleanup(tcl.call, 'encoding', 'system', encoding)
+        tcl.call('encoding', 'system', 'utf-8')
+
+        filename = support.TESTFN
+        self.addCleanup(support.unlink, filename)
+        with open(filename, 'wb') as f:
+            f.write(b"""
+            set a "<\xed\xa0\xbd\xed\xb2\xbb>"
+            set b "<\\ud83d\\udcbb>"
+            """)
+        tcl.evalfile(filename)
+        self.assertEqual(tcl.eval('set a'), '<\U0001f4bb>')
+        self.assertEqual(tcl.eval('set b'), '<\U0001f4bb>')
+
     def testEvalFileException(self):
         tcl = self.interp
         filename = "doesnotexists"
@@ -435,6 +459,11 @@ class TclTest(unittest.TestCase):
         self.assertEqual(passValue('str\x00ing\u20ac'), 'str\x00ing\u20ac')
         self.assertEqual(passValue('str\x00ing\U0001f4bb'),
                          'str\x00ing\U0001f4bb')
+        if sys.platform != 'win32':
+            self.assertEqual(passValue('<\udce2\udc82\udcac>'),
+                             '<\u20ac>')
+            self.assertEqual(passValue('<\udced\udca0\udcbd\udced\udcb2\udcbb>'),
+                             '<\U0001f4bb>')
         self.assertEqual(passValue(b'str\x00ing'),
                          b'str\x00ing' if self.wantobjects else 'str\x00ing')
         self.assertEqual(passValue(b'str\xc0\x80ing'),
@@ -494,6 +523,9 @@ class TclTest(unittest.TestCase):
         check('string\xbd')
         check('string\u20ac')
         check('string\U0001f4bb')
+        if sys.platform != 'win32':
+            check('<\udce2\udc82\udcac>', '<\u20ac>')
+            check('<\udced\udca0\udcbd\udced\udcb2\udcbb>', '<\U0001f4bb>')
         check('')
         check(b'string', 'string')
         check(b'string\xe2\x82\xac', 'string\xe2\x82\xac')
@@ -537,6 +569,8 @@ class TclTest(unittest.TestCase):
             ('a \u20ac', ('a', '\u20ac')),
             ('a \U0001f4bb', ('a', '\U0001f4bb')),
             (b'a \xe2\x82\xac', ('a', '\u20ac')),
+            (b'a \xf0\x9f\x92\xbb', ('a', '\U0001f4bb')),
+            (b'a \xed\xa0\xbd\xed\xb2\xbb', ('a', '\U0001f4bb')),
             (b'a\xc0\x80b c\xc0\x80d', ('a\x00b', 'c\x00d')),
             ('a {b c}', ('a', 'b c')),
             (r'a b\ c', ('a', 'b c')),
diff --git a/Misc/NEWS.d/next/Library/2020-11-14-13-46-27.bpo-42318.wYAcBD.rst b/Misc/NEWS.d/next/Library/2020-11-14-13-46-27.bpo-42318.wYAcBD.rst
new file mode 100644
index 00000000000..e72daebb2f1
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-11-14-13-46-27.bpo-42318.wYAcBD.rst
@@ -0,0 +1 @@
+Fixed support of non-BMP characters in :mod:`tkinter` on macOS.
diff --git a/Modules/_tkinter.c b/Modules/_tkinter.c
index a1071e5a581..c1eb6e10c7f 100644
--- a/Modules/_tkinter.c
+++ b/Modules/_tkinter.c
@@ -397,7 +397,8 @@ unicodeFromTclStringAndSize(const char *s, Py_ssize_t size)
 
     char *buf = NULL;
     PyErr_Clear();
-    /* Tcl encodes null character as \xc0\x80 */
+    /* Tcl encodes null character as \xc0\x80.
+       https://en.wikipedia.org/wiki/UTF-8#Modified_UTF-8 */
     if (memchr(s, '\xc0', size)) {
         char *q;
         const char *e = s + size;
@@ -421,6 +422,57 @@ unicodeFromTclStringAndSize(const char *s, Py_ssize_t size)
     if (buf != NULL) {
         PyMem_Free(buf);
     }
+    if (r == NULL || PyUnicode_KIND(r) == PyUnicode_1BYTE_KIND) {
+        return r;
+    }
+
+    /* In CESU-8 non-BMP characters are represented as a surrogate pair,
+       like in UTF-16, and then each surrogate code point is encoded in UTF-8.
+       https://en.wikipedia.org/wiki/CESU-8 */
+    Py_ssize_t len = PyUnicode_GET_LENGTH(r);
+    Py_ssize_t i, j;
+    /* All encoded surrogate characters start with \xED. */
+    i = PyUnicode_FindChar(r, 0xdcED, 0, len, 1);
+    if (i == -2) {
+        Py_DECREF(r);
+        return NULL;
+    }
+    if (i == -1) {
+        return r;
+    }
+    Py_UCS4 *u = PyUnicode_AsUCS4Copy(r);
+    Py_DECREF(r);
+    if (u == NULL) {
+        return NULL;
+    }
+    Py_UCS4 ch;
+    for (j = i; i < len; i++, u[j++] = ch) {
+        Py_UCS4 ch1, ch2, ch3, high, low;
+        /* Low surrogates U+D800 - U+DBFF are encoded as
+           \xED\xA0\x80 - \xED\xAF\xBF. */
+        ch1 = ch = u[i];
+        if (ch1 != 0xdcED) continue;
+        ch2 = u[i + 1];
+        if (!(0xdcA0 <= ch2 && ch2 <= 0xdcAF)) continue;
+        ch3 = u[i + 2];
+        if (!(0xdc80 <= ch3 && ch3 <= 0xdcBF)) continue;
+        high = 0xD000 | ((ch2 & 0x3F) << 6) | (ch3 & 0x3F);
+        assert(Py_UNICODE_IS_HIGH_SURROGATE(high));
+        /* High surrogates U+DC00 - U+DFFF are encoded as
+           \xED\xB0\x80 - \xED\xBF\xBF. */
+        ch1 = u[i + 3];
+        if (ch1 != 0xdcED) continue;
+        ch2 = u[i + 4];
+        if (!(0xdcB0 <= ch2 && ch2 <= 0xdcBF)) continue;
+        ch3 = u[i + 5];
+        if (!(0xdc80 <= ch3 && ch3 <= 0xdcBF)) continue;
+        low = 0xD000 | ((ch2 & 0x3F) << 6) | (ch3 & 0x3F);
+        assert(Py_UNICODE_IS_HIGH_SURROGATE(high));
+        ch = Py_UNICODE_JOIN_SURROGATES(high, low);
+        i += 5;
+    }
+    r = PyUnicode_FromKindAndData(PyUnicode_4BYTE_KIND, u, j);
+    PyMem_Free(u);
     return r;
 }