diff --git a/Lib/test/test_queue.py b/Lib/test/test_queue.py
index c855fb8fe2b05a..a67f5c51cc00a5 100644
--- a/Lib/test/test_queue.py
+++ b/Lib/test/test_queue.py
@@ -9,6 +9,8 @@
 from test.support import gc_collect, bigmemtest
 from test.support import import_helper
 from test.support import threading_helper
+import sys
+from test import support
 
 # queue module depends on threading primitives
 threading_helper.requires_working_threading(module=True)
@@ -1031,6 +1033,16 @@ def test_is_default(self):
         self.assertIs(self.type2test, self.queue.SimpleQueue)
         self.assertIs(self.type2test, self.queue.SimpleQueue)
 
+    def test_simplequeue_sizeof_reflects_buffer_growth(self):
+        q = self.type2test()
+        before = sys.getsizeof(q)
+        for _ in range(1000):
+            q.put(object())
+        after = sys.getsizeof(q)
+        self.assertGreater(after, before)
+        ptr = support.calcobjsize("P") - support.calcobjsize("")
+        self.assertEqual((after - before) % ptr, 0)
+
     def test_reentrancy(self):
         # bpo-14976: put() may be called reentrantly in an asynchronous
         # callback.
diff --git a/Misc/NEWS.d/next/Library/2025-12-26-09-47-05.gh-issue-140025.zOX58_.rst b/Misc/NEWS.d/next/Library/2025-12-26-09-47-05.gh-issue-140025.zOX58_.rst
new file mode 100644
index 00000000000000..f086e70746946a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-12-26-09-47-05.gh-issue-140025.zOX58_.rst
@@ -0,0 +1 @@
+Fix ``queue.SimpleQueue.__sizeof__()`` computation.
diff --git a/Modules/_queuemodule.c b/Modules/_queuemodule.c
index 01235c77bd7db8..7b17b7ad295b38 100644
--- a/Modules/_queuemodule.c
+++ b/Modules/_queuemodule.c
@@ -239,6 +239,28 @@ simplequeue_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+_queue.SimpleQueue.__sizeof__ -> Py_ssize_t
+
+Returns size in memory, in bytes.
+[clinic start generated code]*/
+
+static Py_ssize_t
+_queue_SimpleQueue___sizeof___impl(simplequeueobject *self)
+{
+    Py_ssize_t size = Py_TYPE(self)->tp_basicsize;
+    PyObject **items = self->buf.items;
+    Py_ssize_t items_cap = self->buf.items_cap;
+
+    if (items != NULL) {
+        size += items_cap * (Py_ssize_t)sizeof(void *);
+    }
+
+    return size;
+}
+/*[clinic end generated code: output=58ce4e3bbc078fd4 input=a3a7f05c9616598f]*/
+
 /*[clinic input]
 @classmethod
 _queue.SimpleQueue.__new__ as simplequeue_new
@@ -534,6 +556,7 @@ static PyMethodDef simplequeue_methods[] = {
     _QUEUE_SIMPLEQUEUE_PUT_METHODDEF
     _QUEUE_SIMPLEQUEUE_PUT_NOWAIT_METHODDEF
     _QUEUE_SIMPLEQUEUE_QSIZE_METHODDEF
+    _QUEUE_SIMPLEQUEUE___SIZEOF___METHODDEF
     {"__class_getitem__",    Py_GenericAlias,
     METH_O|METH_CLASS,       PyDoc_STR("See PEP 585")},
     {NULL,           NULL}              /* sentinel */
diff --git a/Modules/clinic/_queuemodule.c.h b/Modules/clinic/_queuemodule.c.h
index 1751d68716ba5f..97b5542241fb0c 100644
--- a/Modules/clinic/_queuemodule.c.h
+++ b/Modules/clinic/_queuemodule.c.h
@@ -9,6 +9,36 @@ preserve
 #include "pycore_critical_section.h"// Py_BEGIN_CRITICAL_SECTION()
 #include "pycore_modsupport.h"    // _PyArg_NoKeywords()
 
+PyDoc_STRVAR(_queue_SimpleQueue___sizeof____doc__,
+"__sizeof__($self, /)\n"
+"--\n"
+"\n"
+"Returns size in memory, in bytes.");
+
+#define _QUEUE_SIMPLEQUEUE___SIZEOF___METHODDEF    \
+    {"__sizeof__", (PyCFunction)_queue_SimpleQueue___sizeof__, METH_NOARGS, _queue_SimpleQueue___sizeof____doc__},
+
+static Py_ssize_t
+_queue_SimpleQueue___sizeof___impl(simplequeueobject *self);
+
+static PyObject *
+_queue_SimpleQueue___sizeof__(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    PyObject *return_value = NULL;
+    Py_ssize_t _return_value;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    _return_value = _queue_SimpleQueue___sizeof___impl((simplequeueobject *)self);
+    Py_END_CRITICAL_SECTION();
+    if ((_return_value == -1) && PyErr_Occurred()) {
+        goto exit;
+    }
+    return_value = PyLong_FromSsize_t(_return_value);
+
+exit:
+    return return_value;
+}
+
 PyDoc_STRVAR(simplequeue_new__doc__,
 "SimpleQueue()\n"
 "--\n"
@@ -358,4 +388,4 @@ _queue_SimpleQueue_qsize(PyObject *self, PyObject *Py_UNUSED(ignored))
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=1d3efe9df89997cf input=a9049054013a1b77]*/
+/*[clinic end generated code: output=e6a05dc21a1e9fca input=a9049054013a1b77]*/