diff --git a/Lib/test/test_profiling/test_tracing_profiler.py b/Lib/test/test_profiling/test_tracing_profiler.py
index 50de9af86e86d0..7e2ed24f4918e8 100644
--- a/Lib/test/test_profiling/test_tracing_profiler.py
+++ b/Lib/test/test_profiling/test_tracing_profiler.py
@@ -85,6 +85,39 @@ def __call__(self):
             profiler_with_evil_timer.clear()
             self.assertEqual(cm.unraisable.exc_type, RuntimeError)
 
+    def test_external_timer_result_reentry(self):
+        import _lsprof
+
+        class Ticks:
+            def __index__(self):
+                profiler.clear()
+                return 0
+
+        profiler = _lsprof.Profiler(Ticks, timeunit=1)
+        with support.catch_unraisable_exception() as cm:
+            profiler.enable()
+            try:
+                (lambda: None)()
+            finally:
+                profiler.disable()
+            self.assertIs(cm.unraisable.exc_type, RuntimeError)
+
+    def test_external_timer_reinitializes_profiler(self):
+        import _lsprof
+
+        def timer():
+            profiler.__init__()
+            return 0
+
+        profiler = _lsprof.Profiler(timer)
+        with support.catch_unraisable_exception() as cm:
+            profiler.enable()
+            try:
+                (lambda: None)()
+            finally:
+                profiler.disable()
+            self.assertIs(cm.unraisable.exc_type, RuntimeError)
+
     def test_enable_in_external_timer(self):
         # gh-157639: Enabling the profiler from an external timer should not crash
         import _lsprof
diff --git a/Misc/NEWS.d/next/Library/2026-10-22-16-30-00.gh-issue-159739.mFWntK.rst b/Misc/NEWS.d/next/Library/2026-10-22-16-30-00.gh-issue-159739.mFWntK.rst
new file mode 100644
index 00000000000000..73e7f5e0c1c15f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-22-16-30-00.gh-issue-159739.mFWntK.rst
@@ -0,0 +1,3 @@
+Fix crashes in :mod:`profiling.tracing` when a custom timer reinitializes the
+profiler or its result clears or disables the profiler during conversion or
+cleanup.
diff --git a/Modules/_lsprof.c b/Modules/_lsprof.c
index 65857c495f2339..42fd1eaa0f088a 100644
--- a/Modules/_lsprof.c
+++ b/Modules/_lsprof.c
@@ -52,6 +52,7 @@ typedef struct {
     ProfilerContext *currentProfilerContext;
     ProfilerContext *freelistProfilerContext;
     int flags;
+    unsigned int activeTimers;  /* may overlap if the profiler lock is released */
     PyObject *externalTimer;
     double externalTimerUnit;
     int tool_id;
@@ -63,7 +64,6 @@ typedef struct {
 #define POF_ENABLED     0x001
 #define POF_SUBCALLS    0x002
 #define POF_BUILTINS    0x004
-#define POF_EXT_TIMER   0x008
 #define POF_NOMEMORY    0x100
 
 /*[clinic input]
@@ -92,13 +92,7 @@ _lsprof_get_state(PyObject *module)
 
 static PyTime_t CallExternalTimer(ProfilerObject *pObj)
 {
-    PyObject *o = NULL;
-
-    // External timer can do arbitrary things so we need a flag to prevent
-    // horrible things to happen
-    pObj->flags |= POF_EXT_TIMER;
-    o = _PyObject_CallNoArgs(pObj->externalTimer);
-    pObj->flags &= ~POF_EXT_TIMER;
+    PyObject *o = _PyObject_CallNoArgs(pObj->externalTimer);
 
     if (o == NULL) {
         PyErr_FormatUnraisable("Exception ignored while calling "
@@ -132,7 +126,12 @@ static inline PyTime_t
 call_timer(ProfilerObject *pObj)
 {
     if (pObj->externalTimer != NULL) {
-        return CallExternalTimer(pObj);
+        /* Converting or releasing the result and reporting an unraisable
+           exception can all run Python code. */
+        pObj->activeTimers++;
+        PyTime_t result = CallExternalTimer(pObj);
+        pObj->activeTimers--;
+        return result;
     }
     else {
         PyTime_t t;
@@ -365,7 +364,7 @@ ptrace_enter_call(PyObject *self, void *key, PyObject *userObj)
 
     /* Events raised by the external timer must be ignored: it can run
        arbitrary code while a context is still being unwound. */
-    if (pObj->flags & POF_EXT_TIMER) {
+    if (pObj->activeTimers) {
         return;
     }
 
@@ -411,7 +410,7 @@ ptrace_leave_call(PyObject *self, void *key)
     ProfilerEntry *profEntry;
     ProfilerContext *pContext;
 
-    if (pObj->flags & POF_EXT_TIMER) {
+    if (pObj->activeTimers) {
         return;
     }
 
@@ -897,7 +896,7 @@ static PyObject *
 _lsprof_Profiler_disable_impl(ProfilerObject *self)
 /*[clinic end generated code: output=838cffef7f651870 input=f7e4787cae20f7f6]*/
 {
-    if (self->flags & POF_EXT_TIMER) {
+    if (self->activeTimers) {
         PyErr_SetString(PyExc_RuntimeError,
                         "cannot disable profiler in external timer");
         return NULL;
@@ -957,7 +956,7 @@ static PyObject *
 _lsprof_Profiler_clear_impl(ProfilerObject *self)
 /*[clinic end generated code: output=dd1c668fb84b1335 input=4aab219d5d7a9bec]*/
 {
-    if (self->flags & POF_EXT_TIMER) {
+    if (self->activeTimers) {
         PyErr_SetString(PyExc_RuntimeError,
                         "cannot clear profiler in external timer");
         return NULL;
@@ -1003,6 +1002,7 @@ profiler_dealloc(PyObject *op)
 }
 
 /*[clinic input]
+@critical_section
 _lsprof.Profiler.__init__ as profiler_init
 
     timer: object(c_default='NULL') = None
@@ -1021,8 +1021,13 @@ is, in seconds).
 static int
 profiler_init_impl(ProfilerObject *self, PyObject *timer, double timeunit,
                    int subcalls, int builtins)
-/*[clinic end generated code: output=ac523803ec9f9df2 input=8285ca746f96a414]*/
+/*[clinic end generated code: output=ac523803ec9f9df2 input=5add5fc72beb63ba]*/
 {
+    if (self->activeTimers) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "cannot initialize profiler in external timer");
+        return -1;
+    }
     if (setSubcalls(self, subcalls) < 0 || setBuiltins(self, builtins) < 0) {
         return -1;
     }
diff --git a/Modules/clinic/_lsprof.c.h b/Modules/clinic/_lsprof.c.h
index acb4aaf27e377f..0b1913a729ae89 100644
--- a/Modules/clinic/_lsprof.c.h
+++ b/Modules/clinic/_lsprof.c.h
@@ -471,9 +471,11 @@ profiler_init(PyObject *self, PyObject *args, PyObject *kwargs)
         goto exit;
     }
 skip_optional_pos:
+    Py_BEGIN_CRITICAL_SECTION(self);
     return_value = profiler_init_impl((ProfilerObject *)self, timer, timeunit, subcalls, builtins);
+    Py_END_CRITICAL_SECTION();
 
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=af26a0b0ddcc3351 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=82541e3b3fe9819c input=a9049054013a1b77]*/