diff --git a/src/humanize/filesize.py b/src/humanize/filesize.py
index 063affe0..82fc0531 100644
--- a/src/humanize/filesize.py
+++ b/src/humanize/filesize.py
@@ -5,6 +5,7 @@
 __lazy_modules__ = {"humanize.i18n"}
 
 from humanize.i18n import _gettext as _
+from humanize.i18n import decimal_separator
 
 suffixes = {
     "decimal": (
@@ -106,5 +107,10 @@ def naturalsize(
     if exp < len(suffix) and abs(float(format % (abs_bytes / (base**exp)))) >= base:
         exp += 1
     space = "" if gnu else " "
-    ret: str = format % (bytes_ / (base**exp)) + space + _(suffix[exp - 1])
-    return ret
+    formatted = format % (bytes_ / (base**exp))
+    # Default locales keep ".". Only rewrite when a locale uses another
+    # decimal separator so the common path stays a single format + concat.
+    sep = decimal_separator()
+    if sep != ".":
+        formatted = formatted.replace(".", sep, 1)
+    return formatted + space + _(suffix[exp - 1])
diff --git a/tests/test_filesize.py b/tests/test_filesize.py
index 645751b9..4cae6b85 100644
--- a/tests/test_filesize.py
+++ b/tests/test_filesize.py
@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import gettext
+
 import pytest
 
 import humanize
@@ -101,3 +103,19 @@ def test_naturalsize(test_args: list[int] | list[int | bool], expected: str) ->
         test_args[0] = f"-{test_args[0]}"
 
     assert humanize.naturalsize(*test_args) == "-" + expected
+
+
+def test_naturalsize_uses_locale_decimal_separator() -> None:
+    """Locales with a comma radix should not keep a dotted mantissa.
+
+    Uses a NullTranslations catalog so this does not depend on compiled .mo files.
+    """
+    humanize.i18n._TRANSLATIONS["fr_FR"] = gettext.NullTranslations()
+    humanize.i18n._CURRENT.locale = "fr_FR"
+    try:
+        assert humanize.naturalsize(1_500_000) == "1,5 MB"
+        assert humanize.naturalsize(3000, gnu=True) == "2,9K"
+        assert humanize.naturalsize(3000, binary=True) == "2,9 KiB"
+    finally:
+        humanize.i18n.deactivate()
+        assert humanize.naturalsize(1_500_000) == "1.5 MB"
diff --git a/tests/test_i18n.py b/tests/test_i18n.py
index 683fe3ce..1dbc039f 100644
--- a/tests/test_i18n.py
+++ b/tests/test_i18n.py
@@ -241,10 +241,11 @@ def test_intword_i18n(locale: str, number: int, expected_result: str) -> None:
         ("fr_FR", 42, "42 octets"),
         ("si_LK", 1, "බයිට් 1"),
         ("si_LK", 42, "බයිට් 42"),
-        ("fr_FR", 42_000, "42.0 Ko"),
-        ("fr_FR", 42_000_000, "42.0 Mo"),
-        ("fr_FR", 42_000_000_000, "42.0 Go"),
-        ("fr_FR", -42_000, "-42.0 Ko"),
+        ("fr_FR", 42_000, "42,0 Ko"),
+        ("fr_FR", 42_000_000, "42,0 Mo"),
+        ("fr_FR", 42_000_000_000, "42,0 Go"),
+        ("fr_FR", -42_000, "-42,0 Ko"),
+        ("fr_FR", 1_500_000, "1,5 Mo"),
     ],
 )
 def test_naturalsize_i18n(locale: str, value: float, expected_result: str) -> None:
@@ -264,7 +265,7 @@ def test_naturalsize_i18n_binary() -> None:
     except FileNotFoundError:
         pytest.skip("Generate .mo with scripts/generate-translation-binaries.sh")
     else:
-        assert humanize.naturalsize(3000, binary=True) == "2.9 Kio"
+        assert humanize.naturalsize(3000, binary=True) == "2,9 Kio"
     finally:
         humanize.i18n.deactivate()