diff --git a/config/user-largefile.m4 b/config/user-largefile.m4
new file mode 100644
index 000000000..e9e576f86
--- /dev/null
+++ b/config/user-largefile.m4
@@ -0,0 +1,21 @@
+dnl #
+dnl # ZFS_AC_CONFIG_USER_LARGEFILE
+dnl #
+dnl # Ensure off_t is 64-bit for large file support in userspace.
+dnl # This is required for OpenZFS to handle files larger than 2GB.
+dnl #
+AC_DEFUN([ZFS_AC_CONFIG_USER_LARGEFILE], [
+	AC_SYS_LARGEFILE
+	AC_CHECK_SIZEOF([off_t])
+
+	AC_MSG_CHECKING([for 64-bit off_t])
+	AS_IF([test "$ac_cv_sizeof_off_t" -ne 8], [
+		AC_MSG_RESULT([no, $ac_cv_sizeof_off_t bytes])
+		AC_MSG_FAILURE([
+*** OpenZFS userspace requires 64-bit off_t support for large files.
+*** Please ensure your system supports large file operations.
+*** Current off_t size: $ac_cv_sizeof_off_t bytes])
+	], [
+		AC_MSG_RESULT([yes, $ac_cv_sizeof_off_t bytes])
+	])
+])
diff --git a/config/user.m4 b/config/user.m4
index 62e59ed94..28c5162f5 100644
--- a/config/user.m4
+++ b/config/user.m4
@@ -3,6 +3,7 @@ dnl # Default ZFS user configuration
 dnl #
 AC_DEFUN([ZFS_AC_CONFIG_USER], [
 	ZFS_AC_CONFIG_USER_GETTEXT
+	ZFS_AC_CONFIG_USER_LARGEFILE
 	ZFS_AC_CONFIG_USER_MOUNT_HELPER
 	ZFS_AC_CONFIG_USER_SYSVINIT
 	ZFS_AC_CONFIG_USER_DRACUT
diff --git a/include/sys/zfs_file.h b/include/sys/zfs_file.h
index 67abe9988..34870e826 100644
--- a/include/sys/zfs_file.h
+++ b/include/sys/zfs_file.h
@@ -25,6 +25,17 @@
 
 #include <sys/zfs_context.h>
 
+/*
+ * loff_t is a Linux kernel/VFS type. glibc and musl expose it to user
+ * space via <fcntl.h>, but FreeBSD libc does not. For FreeBSD user
+ * space we map loff_t to off_t so the shared interfaces that use the
+ * loff_t name still compile. The FreeBSD kernel gets loff_t from its
+ * own linux-compat headers.
+ */
+#if !defined(_KERNEL) && defined(__FreeBSD__)
+typedef off_t loff_t;
+#endif
+
 #ifndef _KERNEL
 typedef struct zfs_file {
 	int f_fd;
diff --git a/lib/libspl/include/sys/kstat.h b/lib/libspl/include/sys/kstat.h
index 361f24df3..b86737c96 100644
--- a/lib/libspl/include/sys/kstat.h
+++ b/lib/libspl/include/sys/kstat.h
@@ -86,6 +86,6 @@ extern void kstat_delete(kstat_t *);
 extern void kstat_set_raw_ops(kstat_t *ksp,
     int (*headers)(char *buf, size_t size),
     int (*data)(char *buf, size_t size, void *data),
-    void *(*addr)(kstat_t *ksp, loff_t index));
+    void *(*addr)(kstat_t *ksp, off_t index));
 
 #endif	/* _SYS_KSTAT_H */
diff --git a/lib/libspl/include/sys/types.h b/lib/libspl/include/sys/types.h
index 37cf1d241..f4bb85c79 100644
--- a/lib/libspl/include/sys/types.h
+++ b/lib/libspl/include/sys/types.h
@@ -50,19 +50,4 @@ typedef int		projid_t;
 
 #include <sys/param.h> /* for NBBY */
 
-#ifdef __FreeBSD__
-typedef off_t loff_t;
-#endif
-
-/*
- * On musl, loff_t is a macro within fcntl.h when _GNU_SOURCE is defined.
- * If no macro is defined, a typedef fallback is provided.
- */
-#if defined(__linux__) && !defined(__GLIBC__)
-#include <fcntl.h>
-#ifndef loff_t
-typedef off_t loff_t;
-#endif
-#endif
-
 #endif
diff --git a/lib/libspl/kstat.c b/lib/libspl/kstat.c
index af4b870ed..5f62fd358 100644
--- a/lib/libspl/kstat.c
+++ b/lib/libspl/kstat.c
@@ -58,7 +58,7 @@ void
 kstat_set_raw_ops(kstat_t *ksp,
     int (*headers)(char *buf, size_t size),
     int (*data)(char *buf, size_t size, void *data),
-    void *(*addr)(kstat_t *ksp, loff_t index))
+    void *(*addr)(kstat_t *ksp, off_t index))
 {
 	(void) ksp, (void) headers, (void) data, (void) addr;
 }
diff --git a/tests/zfs-tests/cmd/clone_mmap_cached.c b/tests/zfs-tests/cmd/clone_mmap_cached.c
index b0be77798..d10431e3f 100644
--- a/tests/zfs-tests/cmd/clone_mmap_cached.c
+++ b/tests/zfs-tests/cmd/clone_mmap_cached.c
@@ -36,12 +36,13 @@
 #include <string.h>
 #include <unistd.h>
 
-#ifdef __FreeBSD__
-#define	loff_t	off_t
+#if defined(_GNU_SOURCE) && defined(__linux__)
+_Static_assert(sizeof (loff_t) == sizeof (off_t),
+	"loff_t and off_t must be the same size");
 #endif
 
 ssize_t
-copy_file_range(int, loff_t *, int, loff_t *, size_t, unsigned int)
+copy_file_range(int, off_t *, int, off_t *, size_t, unsigned int)
     __attribute__((weak));
 
 static void *
diff --git a/tests/zfs-tests/cmd/clone_mmap_write.c b/tests/zfs-tests/cmd/clone_mmap_write.c
index 8423b6568..183c95d76 100644
--- a/tests/zfs-tests/cmd/clone_mmap_write.c
+++ b/tests/zfs-tests/cmd/clone_mmap_write.c
@@ -42,12 +42,13 @@
 #include <sys/stat.h>
 #include <sys/mman.h>
 
-#ifdef __FreeBSD__
-#define	loff_t	off_t
+#if defined(_GNU_SOURCE) && defined(__linux__)
+_Static_assert(sizeof (loff_t) == sizeof (off_t),
+	"loff_t and off_t must be the same size");
 #endif
 
 ssize_t
-copy_file_range(int, loff_t *, int, loff_t *, size_t, unsigned int)
+copy_file_range(int, off_t *, int, off_t *, size_t, unsigned int)
     __attribute__((weak));
 
 static int
diff --git a/tests/zfs-tests/cmd/clonefile.c b/tests/zfs-tests/cmd/clonefile.c
index 809a33d2f..eae69bef6 100644
--- a/tests/zfs-tests/cmd/clonefile.c
+++ b/tests/zfs-tests/cmd/clonefile.c
@@ -59,16 +59,17 @@
 #endif
 #endif /* __NR_copy_file_range */
 
-#ifdef __FreeBSD__
-#define	loff_t	off_t
+#if defined(_GNU_SOURCE) && defined(__linux__)
+_Static_assert(sizeof (loff_t) == sizeof (off_t),
+	"loff_t and off_t must be the same size");
 #endif
 
 ssize_t
-copy_file_range(int, loff_t *, int, loff_t *, size_t, unsigned int)
+copy_file_range(int, off_t *, int, off_t *, size_t, unsigned int)
     __attribute__((weak));
 
 static inline ssize_t
-cf_copy_file_range(int sfd, loff_t *soff, int dfd, loff_t *doff,
+cf_copy_file_range(int sfd, off_t *soff, int dfd, off_t *doff,
     size_t len, unsigned int flags)
 {
 	if (copy_file_range)
@@ -151,9 +152,9 @@ usage(void)
 }
 
 int do_clone(int sfd, int dfd);
-int do_clonerange(int sfd, int dfd, loff_t soff, loff_t doff, size_t len);
-int do_copyfilerange(int sfd, int dfd, loff_t soff, loff_t doff, size_t len);
-int do_deduperange(int sfd, int dfd, loff_t soff, loff_t doff, size_t len);
+int do_clonerange(int sfd, int dfd, off_t soff, off_t doff, size_t len);
+int do_copyfilerange(int sfd, int dfd, off_t soff, off_t doff, size_t len);
+int do_deduperange(int sfd, int dfd, off_t soff, off_t doff, size_t len);
 
 int quiet = 0;
 
@@ -203,7 +204,7 @@ main(int argc, char **argv)
 			abort();
 	}
 
-	loff_t soff = 0, doff = 0;
+	off_t soff = 0, doff = 0;
 	size_t len = SSIZE_MAX;
 	unsigned long long len2;
 	if ((argc-optind) == 5) {
@@ -295,7 +296,7 @@ do_clone(int sfd, int dfd)
 }
 
 int
-do_clonerange(int sfd, int dfd, loff_t soff, loff_t doff, size_t len)
+do_clonerange(int sfd, int dfd, off_t soff, off_t doff, size_t len)
 {
 	if (!quiet)
 		fprintf(stderr, "using FICLONERANGE\n");
@@ -314,7 +315,7 @@ do_clonerange(int sfd, int dfd, loff_t soff, loff_t doff, size_t len)
 }
 
 int
-do_copyfilerange(int sfd, int dfd, loff_t soff, loff_t doff, size_t len)
+do_copyfilerange(int sfd, int dfd, off_t soff, off_t doff, size_t len)
 {
 	if (!quiet)
 		fprintf(stderr, "using copy_file_range\n");
@@ -341,7 +342,7 @@ do_copyfilerange(int sfd, int dfd, loff_t soff, loff_t doff, size_t len)
 }
 
 int
-do_deduperange(int sfd, int dfd, loff_t soff, loff_t doff, size_t len)
+do_deduperange(int sfd, int dfd, off_t soff, off_t doff, size_t len)
 {
 	if (!quiet)
 		fprintf(stderr, "using FIDEDUPERANGE\n");