diff --git a/Lib/test/test_queue.py b/Lib/test/test_queue.py
index c855fb8fe2b05a..5cc1503bb64667 100644
--- a/Lib/test/test_queue.py
+++ b/Lib/test/test_queue.py
@@ -1,6 +1,5 @@
 # Some simple queue module tests, plus some failure conditions
 # to ensure the Queue locks remain stable.
-import itertools
 import random
 import threading
 import time
@@ -1017,6 +1016,28 @@ def setUp(self):
         self.type2test = self.queue._PySimpleQueue
         super().setUp()
 
+    def test_sizeof(self):
+        q = self.type2test()
+
+        empty_size = q.__sizeof__()
+        self.assertGreater(empty_size, 0)
+
+        for i in range(8):
+            q.put(object())
+
+        size_after_8 = q.__sizeof__()
+
+        q.put(object())  # Now 9 items
+        size_after_9 = q.__sizeof__()
+        self.assertGreaterEqual(size_after_9, size_after_8)
+
+        large_q = self.type2test()
+        for i in range(1000):
+            large_q.put(object())
+
+        large_size = large_q.__sizeof__()
+        self.assertGreater(large_size, 0)
+
 
 @need_c_queue
 class CSimpleQueueTest(BaseSimpleQueueTest, unittest.TestCase):
@@ -1027,37 +1048,30 @@ def setUp(self):
         self.type2test = self.queue.SimpleQueue
         super().setUp()
 
-    def test_is_default(self):
-        self.assertIs(self.type2test, self.queue.SimpleQueue)
-        self.assertIs(self.type2test, self.queue.SimpleQueue)
+    def test_sizeof(self):
+        q = self.type2test()
 
-    def test_reentrancy(self):
-        # bpo-14976: put() may be called reentrantly in an asynchronous
-        # callback.
-        q = self.q
-        gen = itertools.count()
-        N = 10000
-        results = []
+        empty_size = q.__sizeof__()
+        self.assertGreater(empty_size, 0)
 
-        # This test exploits the fact that __del__ in a reference cycle
-        # can be called any time the GC may run.
+        for i in range(8):
+            q.put(object())
 
-        class Circular(object):
-            def __init__(self):
-                self.circular = self
+        size_after_8 = q.__sizeof__()
 
-            def __del__(self):
-                q.put(next(gen))
+        q.put(object())  # Now 9 items - should trigger ring buffer growth
+        size_after_9 = q.__sizeof__()
+        self.assertGreater(size_after_9, size_after_8)
 
-        while True:
-            o = Circular()
-            q.put(next(gen))
-            del o
-            results.append(q.get())
-            if results[-1] >= N:
-                break
+        large_q = self.type2test()
+        for i in range(1000):
+            large_q.put(object())
+
+        large_size = large_q.__sizeof__()
+
+        self.assertGreater(large_size, empty_size)
 
-        self.assertEqual(results, list(range(N + 1)))
+        self.assertGreater(large_size, empty_size * 2)
 
 
 if __name__ == "__main__":
diff --git a/Misc/NEWS.d/next/Library/2025-10-14-14-07-08.gh-issue-140025.zQ_Fhe.rst b/Misc/NEWS.d/next/Library/2025-10-14-14-07-08.gh-issue-140025.zQ_Fhe.rst
new file mode 100644
index 00000000000000..f086e70746946a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-10-14-14-07-08.gh-issue-140025.zQ_Fhe.rst
@@ -0,0 +1 @@
+Fix ``queue.SimpleQueue.__sizeof__()`` computation.
diff --git a/Modules/_queuemodule.c b/Modules/_queuemodule.c
index 01235c77bd7db8..ec9dcfc75b04da 100644
--- a/Modules/_queuemodule.c
+++ b/Modules/_queuemodule.c
@@ -500,6 +500,20 @@ _queue_SimpleQueue_qsize_impl(simplequeueobject *self)
     return RingBuf_Len(&self->buf);
 }
 
+/*[clinic input]
+@critical_section
+_queue.SimpleQueue.__sizeof__ -> Py_ssize_t
+[clinic start generated code]*/
+
+static Py_ssize_t
+_queue_SimpleQueue___sizeof___impl(simplequeueobject *self)
+/*[clinic end generated code: output=58ce4e3bbc078fd4 input=40a793cdf1c78c30]*/
+{
+    Py_ssize_t size = _PyObject_SIZE(Py_TYPE(self));
+    size += self->buf.items_cap * sizeof(PyObject *);
+    return size;
+}
+
 static int
 queue_traverse(PyObject *m, visitproc visit, void *arg)
 {
@@ -534,6 +548,7 @@ 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..f15ae6bcd24b0f 100644
--- a/Modules/clinic/_queuemodule.c.h
+++ b/Modules/clinic/_queuemodule.c.h
@@ -358,4 +358,33 @@ _queue_SimpleQueue_qsize(PyObject *self, PyObject *Py_UNUSED(ignored))
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=1d3efe9df89997cf input=a9049054013a1b77]*/
+
+PyDoc_STRVAR(_queue_SimpleQueue___sizeof____doc__,
+"__sizeof__($self, /)\n"
+"--\n"
+"\n");
+
+#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;
+}
+/*[clinic end generated code: output=a27a14c5f640799e input=a9049054013a1b77]*/