diff --git a/lib/libsys/posix_fallocate.2 b/lib/libsys/posix_fallocate.2
--- a/lib/libsys/posix_fallocate.2
+++ b/lib/libsys/posix_fallocate.2
@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 30, 2020
+.Dd November 2, 2025
 .Dt POSIX_FALLOCATE 2
 .Os
 .Sh NAME
@@ -105,8 +105,7 @@
 .Fa len
 argument was less than or equal to zero, the
 .Fa offset
-argument was less than zero,
-or the operation is not supported by the file system.
+argument was less than zero.
 .It Bq Er EIO
 An I/O error occurred while reading from or writing to a file system.
 .It Bq Er EINTEGRITY
@@ -123,6 +122,8 @@
 The file descriptor
 .Fa fd
 has insufficient rights.
+.It Bq Er EOPNOTSUPP
+The operation is not supported by the file system.
 .It Bq Er ESPIPE
 The
 .Fa fd
@@ -137,12 +138,29 @@
 The
 .Fn posix_fallocate
 system call conforms to
-.St -p1003.1-2004 .
+.St -p1003.1-2024 .
 .Sh HISTORY
 The
 .Fn posix_fallocate
 function appeared in
 .Fx 9.0 .
+.Pp
+Previous versions of
+.Nm
+used
+.Er EINVAL
+to indicate that the operation is not supported by the file system, as specified
+in
+.St -p1003.1
+Base Specifications, Issue 7.
+.St -p1003.1
+Base Specifications, Issue 8 switched to requiring
+.Er EOPNOTSUPP
+for this error case.
+ZFS adopted the latter convention in
+.Fx 15.0 ,
+and the remaining filesystems in base adopted it in
+.Fx 15.1 .
 .Sh AUTHORS
 .Fn posix_fallocate
 and this manual page were initially written by