diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index b2a299ed172967..83def9c31799af 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -1136,6 +1136,18 @@ def test_access_violations(self):
         self.assertEqual(stdout.strip(), b'')
         self.assertEqual(stderr.strip(), b'')
 
+    def test_flush_parameters(self):
+        with open(TESTFN, 'wb+') as f:
+            f.write(b'x' * PAGESIZE * 3)
+            f.flush()
+
+            m = mmap.mmap(f.fileno(), PAGESIZE * 3)
+            self.addCleanup(m.close)
+
+            m.flush()
+            m.flush(PAGESIZE)
+            m.flush(PAGESIZE, PAGESIZE)
+
 
 class LargeMmapTests(unittest.TestCase):
 
diff --git a/Misc/NEWS.d/next/Library/2025-08-24-02-04-32.gh-issue-138092.V4-wTO.rst b/Misc/NEWS.d/next/Library/2025-08-24-02-04-32.gh-issue-138092.V4-wTO.rst
new file mode 100644
index 00000000000000..21c28c49628cba
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-08-24-02-04-32.gh-issue-138092.V4-wTO.rst
@@ -0,0 +1,2 @@
+Fixed a bug in :meth:`mmap.mmap.flush` where calling with only an offset
+parameter would fail.
diff --git a/Modules/clinic/mmapmodule.c.h b/Modules/clinic/mmapmodule.c.h
new file mode 100644
index 00000000000000..ec4282e8082ab7
--- /dev/null
+++ b/Modules/clinic/mmapmodule.c.h
@@ -0,0 +1,70 @@
+/*[clinic input]
+preserve
+[clinic start generated code]*/
+
+#include "pycore_abstract.h"      // _PyNumber_Index()
+#include "pycore_modsupport.h"    // _PyArg_CheckPositional()
+
+PyDoc_STRVAR(mmap_mmap_flush__doc__,
+"flush($self, offset=0, size=None, /)\n"
+"--\n"
+"\n"
+"Flushes changes made to the in-memory copy of a file back to disk.\n"
+"\n"
+"If offset and size are specified, only the specified range will\n"
+"be flushed. If not specified, the entire mapped region will be\n"
+"flushed.");
+
+#define MMAP_MMAP_FLUSH_METHODDEF    \
+    {"flush", _PyCFunction_CAST(mmap_mmap_flush), METH_FASTCALL, mmap_mmap_flush__doc__},
+
+static PyObject *
+mmap_mmap_flush_impl(mmap_object *self, Py_ssize_t offset, Py_ssize_t size);
+
+static PyObject *
+mmap_mmap_flush(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
+{
+    PyObject *return_value = NULL;
+    Py_ssize_t offset = 0;
+    Py_ssize_t size = -1;
+
+    if (!_PyArg_CheckPositional("flush", nargs, 0, 2)) {
+        goto exit;
+    }
+    if (nargs < 1) {
+        goto skip_optional;
+    }
+    {
+        Py_ssize_t ival = -1;
+        PyObject *iobj = _PyNumber_Index(args[0]);
+        if (iobj != NULL) {
+            ival = PyLong_AsSsize_t(iobj);
+            Py_DECREF(iobj);
+        }
+        if (ival == -1 && PyErr_Occurred()) {
+            goto exit;
+        }
+        offset = ival;
+    }
+    if (nargs < 2) {
+        goto skip_optional;
+    }
+    {
+        Py_ssize_t ival = -1;
+        PyObject *iobj = _PyNumber_Index(args[1]);
+        if (iobj != NULL) {
+            ival = PyLong_AsSsize_t(iobj);
+            Py_DECREF(iobj);
+        }
+        if (ival == -1 && PyErr_Occurred()) {
+            goto exit;
+        }
+        size = ival;
+    }
+skip_optional:
+    return_value = mmap_mmap_flush_impl((mmap_object *)self, offset, size);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=c95c7ffe05f26c3a input=a9049054013a1b77]*/
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 0cb4b62d734550..f4d739320952f1 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -28,6 +28,12 @@
 #include "pycore_fileutils.h"     // _Py_stat_struct
 #include "pycore_weakref.h"       // FT_CLEAR_WEAKREFS()
 
+/*[clinic input]
+module mmap
+class mmap.mmap "mmap_object *" "mmap_object_type"
+[clinic start generated code]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=4ebde54549b9daa7]*/
+
 #include <stddef.h>               // offsetof()
 #ifndef MS_WINDOWS
 #  include <unistd.h>             // close()
@@ -128,6 +134,8 @@ typedef struct {
 
 #define mmap_object_CAST(op)    ((mmap_object *)(op))
 
+#include "clinic/mmapmodule.c.h"
+
 static int
 mmap_object_traverse(PyObject *op, visitproc visit, void *arg)
 {
@@ -924,15 +932,31 @@ mmap_tell_method(PyObject *op, PyObject *Py_UNUSED(ignored))
     return PyLong_FromSize_t(self->pos);
 }
 
+/*[clinic input]
+mmap.mmap.flush
+
+    offset: Py_ssize_t = 0
+    size: Py_ssize_t(c_default="-1") = None
+    /
+
+Flushes changes made to the in-memory copy of a file back to disk.
+
+If offset and size are specified, only the specified range will
+be flushed. If not specified, the entire mapped region will be
+flushed.
+[clinic start generated code]*/
+
 static PyObject *
-mmap_flush_method(PyObject *op, PyObject *args)
+mmap_mmap_flush_impl(mmap_object *self, Py_ssize_t offset, Py_ssize_t size)
+/*[clinic end generated code: output=956ced67466149cf input=07c2c6d4e69263a4]*/
 {
-    Py_ssize_t offset = 0;
-    mmap_object *self = mmap_object_CAST(op);
-    Py_ssize_t size = self->size;
     CHECK_VALID(NULL);
-    if (!PyArg_ParseTuple(args, "|nn:flush", &offset, &size))
-        return NULL;
+
+    // If size is -1 (default), calculate size from offset to end.
+    if (size == -1) {
+        size = self->size - offset;
+    }
+
     if (size < 0 || offset < 0 || self->size - offset < size) {
         PyErr_SetString(PyExc_ValueError, "flush values out of range");
         return NULL;
@@ -1194,7 +1218,7 @@ static struct PyMethodDef mmap_object_methods[] = {
     {"close",           mmap_close_method,        METH_NOARGS},
     {"find",            mmap_find_method,         METH_VARARGS},
     {"rfind",           mmap_rfind_method,        METH_VARARGS},
-    {"flush",           mmap_flush_method,        METH_VARARGS},
+    MMAP_MMAP_FLUSH_METHODDEF
 #ifdef HAVE_MADVISE
     {"madvise",         mmap_madvise_method,      METH_VARARGS},
 #endif