diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index b2a299ed172967..dff8d056aa375e 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -506,6 +506,8 @@ def test_anonymous(self):
             m[x] = b
             self.assertEqual(m[x], b)
 
+        self.assertEqual(m.size(), PAGESIZE)
+
     def test_read_all(self):
         m = mmap.mmap(-1, 16)
         self.addCleanup(m.close)
diff --git a/Misc/NEWS.d/next/Library/2025-08-28-01-29-01.gh-issue-87595.Ncgrwo.rst b/Misc/NEWS.d/next/Library/2025-08-28-01-29-01.gh-issue-87595.Ncgrwo.rst
new file mode 100644
index 00000000000000..d978133992068c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-08-28-01-29-01.gh-issue-87595.Ncgrwo.rst
@@ -0,0 +1 @@
+Fix :meth:`mmap.mmap.size` method for anonymous mmap files on Unix.
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index 0cb4b62d734550..0ef91861d7d6e0 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -747,6 +747,9 @@ mmap_size_method(PyObject *op, PyObject *Py_UNUSED(ignored))
 
 #ifdef UNIX
     {
+        if (self->trackfd && self->fd == -1) {
+            return PyLong_FromSsize_t(self->size);
+        }
         struct _Py_stat_struct status;
         if (_Py_fstat(self->fd, &status) == -1)
             return NULL;