diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index ccf85e627f9b5f..ae16bc6990c170 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -1353,6 +1353,32 @@ All of the following functions must be called after :c:func:`Py_Initialize`.
    .. versionadded:: 3.11
 
 
+.. c:function:: int PyUnstable_ThreadState_SetStack(PyThreadState *tstate, void *stack_start_addr, size_t stack_size)
+
+   Set the stack start address and stack size of a Python thread state.
+
+   *stack_size* must be greater than ``0``.
+
+   On success, return ``0``.
+   On failure, set an exception and return ``-1``.
+
+   .. seealso::
+      The :c:func:`PyUnstable_ThreadState_ResetStack` function.
+
+   .. versionadded:: next
+
+
+.. c:function:: void PyUnstable_ThreadState_ResetStack(PyThreadState *tstate)
+
+   Reset the stack start address and stack size of a Python thread state to
+   the operating system defaults.
+
+   .. seealso::
+      The :c:func:`PyUnstable_ThreadState_SetStack` function.
+
+   .. versionadded:: next
+
+
 .. c:function:: PyInterpreterState* PyInterpreterState_Get(void)
 
    Get the current interpreter.
diff --git a/Doc/whatsnew/3.14.rst b/Doc/whatsnew/3.14.rst
index c226f57e5025f0..2582673f0c5c08 100644
--- a/Doc/whatsnew/3.14.rst
+++ b/Doc/whatsnew/3.14.rst
@@ -2981,6 +2981,11 @@ New features in the C API
   as arguments to C API functions.
   (Contributed by Sam Gross in :gh:`133164`.)
 
+* Add :c:func:`PyUnstable_ThreadState_SetStack` and
+  :c:func:`PyUnstable_ThreadState_ResetStack` functions to set the stack base
+  address and stack size of a Python thread state.
+  (Contributed by Victor Stinner in :gh:`139653`.)
+
 
 Limited C API changes
 ---------------------
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index ac8798ff6129a0..62a2fdda52ca75 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -252,6 +252,16 @@ PyAPI_FUNC(int) PyGILState_Check(void);
 */
 PyAPI_FUNC(PyObject*) _PyThread_CurrentFrames(void);
 
+// Set the stack start address and stack size of a Python thread state
+PyAPI_FUNC(int) PyUnstable_ThreadState_SetStack(
+    PyThreadState *tstate,
+    void *stack_start_addr,  // Stack start address
+    size_t stack_size);      // Stack size (in bytes)
+
+// Reset the stack start address and stack size of a Python thread state
+PyAPI_FUNC(void) PyUnstable_ThreadState_ResetStack(
+    PyThreadState *tstate);
+
 /* Routines for advanced debuggers, requested by David Beazley.
    Don't use unless you know what you are doing! */
 PyAPI_FUNC(PyInterpreterState *) PyInterpreterState_Main(void);
diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
index 102a378f8f08bc..908d8f5d8e929b 100644
--- a/Include/internal/pycore_ceval.h
+++ b/Include/internal/pycore_ceval.h
@@ -243,8 +243,6 @@ static inline void _Py_LeaveRecursiveCallTstate(PyThreadState *tstate) {
     (void)tstate;
 }
 
-PyAPI_FUNC(void) _Py_InitializeRecursionLimits(PyThreadState *tstate);
-
 static inline int _Py_ReachedRecursionLimit(PyThreadState *tstate)  {
     uintptr_t here_addr = _Py_get_machine_stack_pointer();
     _PyThreadStateImpl *_tstate = (_PyThreadStateImpl *)tstate;
diff --git a/Misc/NEWS.d/next/C_API/2025-10-06-22-17-47.gh-issue-139653.6-1MOd.rst b/Misc/NEWS.d/next/C_API/2025-10-06-22-17-47.gh-issue-139653.6-1MOd.rst
new file mode 100644
index 00000000000000..f50a976574038f
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2025-10-06-22-17-47.gh-issue-139653.6-1MOd.rst
@@ -0,0 +1,3 @@
+Add :c:func:`PyUnstable_ThreadState_SetStack` and
+:c:func:`PyUnstable_ThreadState_ResetStack` functions to set the stack base
+address and stack size of a Python thread state. Patch by Victor Stinner.
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index c2647d405e25bc..979e098a20a08c 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -2418,6 +2418,41 @@ set_vectorcall_nop(PyObject *self, PyObject *func)
     Py_RETURN_NONE;
 }
 
+static void
+check_threadstate_set_stack(PyThreadState *tstate, void *start, size_t size)
+{
+    assert(PyUnstable_ThreadState_SetStack(tstate, start, size) == 0);
+    assert(!PyErr_Occurred());
+
+    _PyThreadStateImpl *ts = (_PyThreadStateImpl *)tstate;
+    assert(ts->c_stack_hard_limit == (uintptr_t)start);
+    assert(ts->c_stack_top == (uintptr_t)start + size);
+    assert(ts->c_stack_soft_limit >= ts->c_stack_hard_limit);
+    assert(ts->c_stack_soft_limit < ts->c_stack_top);
+}
+
+
+static PyObject *
+test_threadstate_set_stack(PyObject *self, PyObject *Py_UNUSED(args))
+{
+    PyThreadState *tstate = PyThreadState_GET();
+    assert(!PyErr_Occurred());
+
+    // Test a size smaller than _PyOS_STACK_MARGIN_BYTES
+    size_t size = 4096;
+    assert(size < _PyOS_STACK_MARGIN_BYTES);
+    void *start = (void*)(_Py_get_machine_stack_pointer() - size);
+    check_threadstate_set_stack(tstate, start, size);
+
+    // Test a larger size
+    size = 7654321;
+    start = (void*)(_Py_get_machine_stack_pointer() - size);
+    check_threadstate_set_stack(tstate, start, size);
+
+    PyUnstable_ThreadState_ResetStack(tstate);
+    Py_RETURN_NONE;
+}
+
 static PyMethodDef module_functions[] = {
     {"get_configs", get_configs, METH_NOARGS},
     {"get_recursion_depth", get_recursion_depth, METH_NOARGS},
@@ -2527,6 +2562,7 @@ static PyMethodDef module_functions[] = {
 #endif
     {"simple_pending_call", simple_pending_call, METH_O},
     {"set_vectorcall_nop", set_vectorcall_nop, METH_O},
+    {"test_threadstate_set_stack", test_threadstate_set_stack, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };
 
diff --git a/Python/ceval.c b/Python/ceval.c
index 0ccaacaf3ed5b1..5393893650f9d9 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -351,7 +351,7 @@ _Py_ReachedRecursionLimitWithMargin(PyThreadState *tstate, int margin_count)
         return 0;
     }
     if (_tstate->c_stack_hard_limit == 0) {
-        _Py_InitializeRecursionLimits(tstate);
+        PyUnstable_ThreadState_ResetStack(tstate);
     }
     return here_addr <= _tstate->c_stack_soft_limit + margin_count * _PyOS_STACK_MARGIN_BYTES;
 }
@@ -439,35 +439,83 @@ int pthread_attr_destroy(pthread_attr_t *a)
 #endif
 
 
+static void
+tstate_set_stack(PyThreadState *tstate,
+                 void *stack_start_addr, size_t stack_size)
+{
+    assert(stack_size > 0);
+
+    _PyThreadStateImpl *ts = (_PyThreadStateImpl *)tstate;
+    ts->c_stack_hard_limit = (uintptr_t)stack_start_addr;
+    ts->c_stack_top = (uintptr_t)stack_start_addr + stack_size;
+
+    uintptr_t soft_limit = ts->c_stack_hard_limit;
+    if (stack_size >= _PyOS_STACK_MARGIN_BYTES) {
+#ifdef _Py_THREAD_SANITIZER
+        // Thread sanitizer crashes if we use a bit more than half the stack.
+        soft_limit += (stack_size / 2);
+#else
+        soft_limit += _PyOS_STACK_MARGIN_BYTES;
+#endif
+    }
+    ts->c_stack_soft_limit = soft_limit;
+
+    // Sanity checks
+    assert(ts->c_stack_hard_limit <= ts->c_stack_soft_limit);
+    assert(ts->c_stack_soft_limit < ts->c_stack_top);
+
+    // Test the stack pointer
+#if !defined(NDEBUG) && !defined(__wasi__)
+    uintptr_t here_addr = _Py_get_machine_stack_pointer();
+    assert(ts->c_stack_soft_limit < here_addr);
+    assert(here_addr < ts->c_stack_top);
+#endif
+}
+
+
+int
+PyUnstable_ThreadState_SetStack(PyThreadState *tstate,
+                                void *stack_start_addr, size_t stack_size)
+{
+    if (stack_size == 0) {
+        PyErr_SetString(PyExc_ValueError, "stack_size must be greater than 0");
+        return -1;
+    }
+
+    tstate_set_stack(tstate, stack_start_addr, stack_size);
+    return 0;
+}
+
+
 void
-_Py_InitializeRecursionLimits(PyThreadState *tstate)
+PyUnstable_ThreadState_ResetStack(PyThreadState *tstate)
 {
-    _PyThreadStateImpl *_tstate = (_PyThreadStateImpl *)tstate;
 #ifdef WIN32
     ULONG_PTR low, high;
     GetCurrentThreadStackLimits(&low, &high);
-    _tstate->c_stack_top = (uintptr_t)high;
+
     ULONG guarantee = 0;
     SetThreadStackGuarantee(&guarantee);
-    _tstate->c_stack_hard_limit = ((uintptr_t)low) + guarantee + _PyOS_STACK_MARGIN_BYTES;
-    _tstate->c_stack_soft_limit = _tstate->c_stack_hard_limit + _PyOS_STACK_MARGIN_BYTES;
+
+    uintptr_t start = (uintptr_t)low + guarantee + _PyOS_STACK_MARGIN_BYTES;
+    size_t size = (uintptr_t)high - start;
+    tstate_set_stack(tstate, (void*)start, size);
+
 #elif defined(__APPLE__)
     pthread_t this_thread = pthread_self();
-    void *stack_addr = pthread_get_stackaddr_np(this_thread); // top of the stack
-    size_t stack_size = pthread_get_stacksize_np(this_thread);
-    _tstate->c_stack_top = (uintptr_t)stack_addr;
-    _tstate->c_stack_hard_limit = _tstate->c_stack_top - stack_size;
-    _tstate->c_stack_soft_limit = _tstate->c_stack_hard_limit + _PyOS_STACK_MARGIN_BYTES;
+    void *top = pthread_get_stackaddr_np(this_thread); // top of the stack
+    size_t size = pthread_get_stacksize_np(this_thread);
+    tstate_set_stack(tstate, (char*)top - size, size);
+
 #else
-    uintptr_t here_addr = _Py_get_machine_stack_pointer();
-/// XXX musl supports HAVE_PTHRED_GETATTR_NP, but the resulting stack size
-/// (on alpine at least) is much smaller than expected and imposes undue limits
-/// compared to the old stack size estimation.  (We assume musl is not glibc.)
+   // XXX musl supports HAVE_PTHRED_GETATTR_NP, but the resulting stack size
+   // (on alpine at least) is much smaller than expected and imposes undue limits
+   // compared to the old stack size estimation.  (We assume musl is not glibc.)
 #  if defined(HAVE_PTHREAD_GETATTR_NP) && !defined(_AIX) && \
         !defined(__NetBSD__) && (defined(__GLIBC__) || !defined(__linux__))
-    size_t stack_size, guard_size;
-    void *stack_addr;
     pthread_attr_t attr;
+    size_t guard_size, stack_size;
+    void *stack_addr;
     int err = pthread_getattr_np(pthread_self(), &attr);
     if (err == 0) {
         err = pthread_attr_getguardsize(&attr, &guard_size);
@@ -476,25 +524,23 @@ _Py_InitializeRecursionLimits(PyThreadState *tstate)
     }
     if (err == 0) {
         uintptr_t base = ((uintptr_t)stack_addr) + guard_size;
-        _tstate->c_stack_top = base + stack_size;
-#ifdef _Py_THREAD_SANITIZER
-        // Thread sanitizer crashes if we use a bit more than half the stack.
-        _tstate->c_stack_soft_limit = base + (stack_size / 2);
-#else
-        _tstate->c_stack_soft_limit = base + _PyOS_STACK_MARGIN_BYTES * 2;
-#endif
-        _tstate->c_stack_hard_limit = base + _PyOS_STACK_MARGIN_BYTES;
-        assert(_tstate->c_stack_soft_limit < here_addr);
-        assert(here_addr < _tstate->c_stack_top);
-        return;
+        uintptr_t start = base + _PyOS_STACK_MARGIN_BYTES;
+        size_t pystack_size = (base + stack_size) - start;
+        tstate_set_stack(tstate, (void*)start, pystack_size);
     }
+    else
 #  endif
-    _tstate->c_stack_top = _Py_SIZE_ROUND_UP(here_addr, 4096);
-    _tstate->c_stack_soft_limit = _tstate->c_stack_top - Py_C_STACK_SIZE;
-    _tstate->c_stack_hard_limit = _tstate->c_stack_top - (Py_C_STACK_SIZE + _PyOS_STACK_MARGIN_BYTES);
+    {
+        uintptr_t here_addr = _Py_get_machine_stack_pointer();
+        uintptr_t top = _Py_SIZE_ROUND_UP(here_addr, 4096);
+        uintptr_t start = top - (Py_C_STACK_SIZE + _PyOS_STACK_MARGIN_BYTES);
+        size_t pystack_size = top - start;
+        tstate_set_stack(tstate, (void*)start, pystack_size);
+    }
 #endif
 }
 
+
 /* The function _Py_EnterRecursiveCallTstate() only calls _Py_CheckRecursiveCall()
    if the recursion_depth reaches recursion_limit. */
 int
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index b813166f167d70..a4e56c8e5d442c 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -868,7 +868,7 @@ pycore_interp_init(PyThreadState *tstate)
 {
     _PyThreadStateImpl *_tstate = (_PyThreadStateImpl *)tstate;
     if (_tstate->c_stack_hard_limit == 0) {
-        _Py_InitializeRecursionLimits(tstate);
+        PyUnstable_ThreadState_ResetStack(tstate);
     }
     PyInterpreterState *interp = tstate->interp;
     PyStatus status;
diff --git a/Python/pystate.c b/Python/pystate.c
index dbed609f29aa07..d338db7818f341 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -2093,7 +2093,7 @@ _PyThreadState_Attach(PyThreadState *tstate)
     }
     _PyThreadStateImpl *_tstate = (_PyThreadStateImpl *)tstate;
     if (_tstate->c_stack_hard_limit == 0) {
-        _Py_InitializeRecursionLimits(tstate);
+        PyUnstable_ThreadState_ResetStack(tstate);
     }
 
     while (1) {