diff --git a/babel/dates.py b/babel/dates.py
index 69610a7f0..3727b8d19 100644
--- a/babel/dates.py
+++ b/babel/dates.py
@@ -1646,7 +1646,10 @@ 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_dates.py b/tests/test_dates.py
index 12bb23433..b217deab9 100644
--- a/tests/test_dates.py
+++ b/tests/test_dates.py
@@ -177,6 +177,14 @@ def test_fractional_seconds_zero(self):
         t = time(15, 30, 0)
         assert dates.DateTimeFormat(t, locale='en_US')['SSSS'] == '0000'
 
+    def test_fractional_seconds_rounding_does_not_overflow_field(self):
+        t = time(1, 2, 3, 990000)
+        assert dates.DateTimeFormat(t, locale='en_US')['S'] == '9'
+        t = time(1, 2, 3, 999500)
+        assert dates.DateTimeFormat(t, locale='en_US')['SS'] == '99'
+        t = time(1, 2, 3, 999999)
+        assert dates.DateTimeFormat(t, locale='en_US')['SSSS'] == '9999'
+
     def test_milliseconds_in_day(self):
         t = time(15, 30, 12, 345000)
         assert dates.DateTimeFormat(t, locale='en_US')['AAAA'] == '55812345'