diff --git a/libio/fileops.c b/libio/fileops.c
index b00a5d24ab..1a192b7512 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -859,17 +859,21 @@ libc_hidden_ver (_IO_new_file_sync, _IO_file_sync)
 int
 _IO_file_sync_mmap (FILE *fp)
 {
+  off64_t o = fp->_offset - (fp->_IO_read_end - fp->_IO_read_ptr);
   if (fp->_IO_read_ptr != fp->_IO_read_end)
     {
-      if (__lseek64 (fp->_fileno, fp->_IO_read_ptr - fp->_IO_buf_base,
-		     SEEK_SET)
-	  != fp->_IO_read_ptr - fp->_IO_buf_base)
+      if (_IO_in_backup (fp))
+	{
+	  _IO_switch_to_main_get_area (fp);
+	  o -= fp->_IO_read_end - fp->_IO_read_base;
+	}
+      if (__lseek64 (fp->_fileno, o, SEEK_SET) != o)
 	{
 	  fp->_flags |= _IO_ERR_SEEN;
 	  return EOF;
 	}
     }
-  fp->_offset = fp->_IO_read_ptr - fp->_IO_buf_base;
+  fp->_offset = o;
   fp->_IO_read_end = fp->_IO_read_ptr = fp->_IO_read_base;
   return 0;
 }
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index f80b9ec57b..2282ea3bf7 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -239,6 +239,7 @@ tests := \
   tst-fdopen2 \
   tst-ferror \
   tst-fflush-all-input \
+  tst-fflush-mmap \
   tst-fgets \
   tst-fgets2 \
   tst-fileno \
diff --git a/stdio-common/tst-fflush-mmap.c b/stdio-common/tst-fflush-mmap.c
new file mode 100644
index 0000000000..3bddb909e0
--- /dev/null
+++ b/stdio-common/tst-fflush-mmap.c
@@ -0,0 +1,50 @@
+/* Test fflush after ungetc on files using mmap (bug 32535).
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+
+#include <support/check.h>
+#include <support/temp_file.h>
+#include <support/xstdio.h>
+#include <support/xunistd.h>
+
+int
+do_test (void)
+{
+  char *filename = NULL;
+  int fd = create_temp_file ("tst-fflush-mmap", &filename);
+  TEST_VERIFY_EXIT (fd != -1);
+  xclose (fd);
+
+  /* Test fflush after ungetc (bug 32535).  */
+  FILE *fp = xfopen (filename, "w");
+  TEST_VERIFY (0 <= fputs ("test", fp));
+  xfclose (fp);
+
+  fp = xfopen (filename, "rm");
+  TEST_COMPARE (fgetc (fp), 't');
+  TEST_COMPARE (ungetc ('u', fp), 'u');
+  TEST_COMPARE (fflush (fp), 0);
+  TEST_COMPARE (fgetc (fp), 't');
+  TEST_COMPARE (fgetc (fp), 'e');
+  xfclose (fp);
+
+  return 0;
+}
+
+#include <support/test-driver.c>