diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index f169513c..b131ab28 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -1,3 +1,8 @@
+v4.27.0
+=======
+
+* Report a string as an invalid ``regex`` format rather than crashing when it contains a syntactically valid but oversized repetition count (which makes ``re.compile`` raise ``OverflowError`` instead of ``re.error``).
+
 v4.26.0
 =======
 
diff --git a/jsonschema/_format.py b/jsonschema/_format.py
index 62c0e4ee..ca7b5451 100644
--- a/jsonschema/_format.py
+++ b/jsonschema/_format.py
@@ -413,7 +413,7 @@ def is_time(instance: object) -> bool:
         return is_datetime("1970-01-01T" + instance)
 
 
-@_checks_drafts(name="regex", raises=re.error)
+@_checks_drafts(name="regex", raises=(re.error, OverflowError))
 def is_regex(instance: object) -> bool:
     if not isinstance(instance, str):
         return True
diff --git a/jsonschema/tests/test_format.py b/jsonschema/tests/test_format.py
index d829f984..536fadfc 100644
--- a/jsonschema/tests/test_format.py
+++ b/jsonschema/tests/test_format.py
@@ -80,6 +80,15 @@ def test_format_checkers_come_with_defaults(self):
         with self.assertRaises(FormatError):
             checker.check(instance="not-an-ipv4", format="ipv4")
 
+    def test_regex_format_rejects_overflowing_repeat_count(self):
+        # A syntactically valid but oversized repetition count makes
+        # re.compile raise OverflowError, which is not an re.error, so it
+        # must be declared alongside it or it escapes uncaught instead of
+        # being reported as an invalid regex.
+        checker = FormatChecker()
+        with self.assertRaises(FormatError):
+            checker.check(instance="a{99999999999999}", format="regex")
+
     def test_repr(self):
         checker = FormatChecker(formats=())
         checker.checks("foo")(lambda thing: True)  # pragma: no cover