diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 6a646a07911..6680f7433ab 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -943,6 +943,8 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertEqual("{0:.0s}".format("ABC\u0410\u0411\u0412"),
                          '')
 
+        self.assertEqual("{[{}]}".format({"{}": 5}), "5")
+
     def test_format_map(self):
         self.assertEqual(''.format_map({}), '')
         self.assertEqual('a'.format_map({}), 'a')
diff --git a/Misc/NEWS b/Misc/NEWS
index 716378aea14..68902e5c1fe 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -13,6 +13,9 @@ Core and Builtins
 - Issue #12370: Prevent class bodies from interfering with the __class__
   closure.
 
+- Issue #17644: Fix a crash in str.format when curly braces are used in square
+  brackets.
+
 - Issue #17237: Fix crash in the ASCII decoder on m68k.
 
 - Issue #17927: Frame objects kept arguments alive if they had been
diff --git a/Objects/stringlib/unicode_format.h b/Objects/stringlib/unicode_format.h
index 942916938c8..b01d756e71b 100644
--- a/Objects/stringlib/unicode_format.h
+++ b/Objects/stringlib/unicode_format.h
@@ -638,7 +638,7 @@ MarkupIterator_next(MarkupIterator *self, SubString *literal,
                     SubString *format_spec, Py_UCS4 *conversion,
                     int *format_spec_needs_expanding)
 {
-    int at_end;
+    int at_end, hit_format_spec;
     Py_UCS4 c = 0;
     Py_ssize_t start;
     int count;
@@ -723,12 +723,18 @@ MarkupIterator_next(MarkupIterator *self, SubString *literal,
 
     /* we know we can't have a zero length string, so don't worry
        about that case */
+    hit_format_spec = 0;
     while (self->str.start < self->str.end) {
         switch (c = PyUnicode_READ_CHAR(self->str.str, self->str.start++)) {
+        case ':':
+            hit_format_spec = 1;
+            count = 1;
+            break;
         case '{':
             /* the format spec needs to be recursively expanded.
                this is an optimization, and not strictly needed */
-            *format_spec_needs_expanding = 1;
+            if (hit_format_spec)
+                *format_spec_needs_expanding = 1;
             count++;
             break;
         case '}':