diff --git a/babel/dates.py b/babel/dates.py
index 7b626aa11..37c48235e 100644
--- a/babel/dates.py
+++ b/babel/dates.py
@@ -1638,7 +1638,10 @@ def format_frac_seconds(self, num: int) -> str:
         of digits passed in.
         """
         value = self.value.microsecond / 1000000
-        return self.format(round(value, num) * 10**num, num)
+        # Rounding can carry the value up to a whole second (for example 0.999
+        # rounds to 1.0), which would add a digit; clamp to the field width.
+        frac = min(int(round(value, num) * 10**num), 10**num - 1)
+        return self.format(frac, num)
 
     def format_milliseconds_in_day(self, num):
         msecs = (
diff --git a/tests/test_date_time_format.py b/tests/test_date_time_format.py
index 5ca620879..a60dc2242 100644
--- a/tests/test_date_time_format.py
+++ b/tests/test_date_time_format.py
@@ -175,6 +175,15 @@ def test_fractional_seconds():
     assert DateTimeFormat(t, locale='en_US')['SSSSS'] == '00080'
 
 
+def test_fractional_seconds_rounding_does_not_overflow_field():
+    t = time(1, 2, 3, 990000)
+    assert DateTimeFormat(t, locale='en_US')['S'] == '9'
+    t = time(1, 2, 3, 999500)
+    assert DateTimeFormat(t, locale='en_US')['SS'] == '99'
+    t = time(1, 2, 3, 999999)
+    assert DateTimeFormat(t, locale='en_US')['SSSS'] == '9999'
+
+
 def test_fractional_seconds_zero():
     t = time(15, 30, 0)
     assert DateTimeFormat(t, locale='en_US')['SSSS'] == '0000'