diff --git a/src/humanize/filesize.py b/src/humanize/filesize.py
index c495fed..315261a 100644
--- a/src/humanize/filesize.py
+++ b/src/humanize/filesize.py
@@ -97,6 +97,12 @@ def naturalsize(
         return f"{int(bytes_)}B" if gnu else _("%d Bytes") % int(bytes_)
 
     exp = int(min(log(abs_bytes, base), len(suffix)))
+    # The suffix is chosen from the unrounded byte count, but `format` rounds the
+    # mantissa afterward; rounding can push it up to `base` (e.g. 999999 is
+    # 999.999 kB, which formats to "1000.0 kB"). When that happens and a larger
+    # suffix is available, step up one suffix so the result reads "1.0 MB".
+    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
diff --git a/tests/test_filesize.py b/tests/test_filesize.py
index 04774d9..e695639 100644
--- a/tests/test_filesize.py
+++ b/tests/test_filesize.py
@@ -82,6 +82,15 @@
         ([1.123456789, False, True], "1B"),
         ([1.123456789 * 10**3, False, True], "1.1K"),
         ([1.123456789 * 10**6, False, True], "1.1M"),
+        # Rounding must not leave the mantissa at the base while a larger suffix
+        # is available: 999999 is 999.999 kB, which the "%.1f" format rounds to
+        # 1000.0 and must carry into 1.0 MB rather than render as "1000.0 kB".
+        ([999999], "1.0 MB"),
+        ([999999999], "1.0 GB"),
+        ([999999999999], "1.0 TB"),
+        ([1024**2 - 1, True], "1.0 MiB"),
+        ([1024**3 - 1, True], "1.0 GiB"),
+        ([1024**2 - 1, False, True], "1.0M"),
     ],
 )
 def test_naturalsize(test_args: list[int] | list[int | bool], expected: str) -> None: