diff --git a/sentry_sdk/sessions.py b/sentry_sdk/sessions.py
index a5dd589ee9..2fc4abb407 100644
--- a/sentry_sdk/sessions.py
+++ b/sentry_sdk/sessions.py
@@ -165,8 +165,9 @@ def __init__(
 
     def flush(self):
         # type: (...) -> None
-        pending_sessions = self.pending_sessions
-        self.pending_sessions = []
+        with self._thread_lock:
+            pending_sessions = self.pending_sessions
+            self.pending_sessions = []
 
         with self._aggregate_lock:
             pending_aggregates = self.pending_aggregates
@@ -190,14 +191,34 @@ def flush(self):
         if len(envelope.items) > 0:
             self.capture_func(envelope)
 
+        # hygiene: deterministically clean up any stopping thread
+        if not self._should_join_thread():
+            return
+        with self._thread_lock:
+            if not self._should_join_thread():
+                return
+            if self._thread:  # typing
+                self._thread.join()
+                self._thread = None
+                self._thread_for_pid = None
+
+    def _should_join_thread(self):
+        # type: (...) -> bool
+        return (
+            not self.__shutdown_requested.is_set()
+            and self._thread is not None
+            # we are the parent thread:
+            and self._thread_for_pid == os.getpid()
+        )
+
     def _ensure_running(self):
         # type: (...) -> None
         """
         Check that we have an active thread to run in, or create one if not.
 
         Note that this might fail (e.g. in Python 3.12 it's not possible to
-        spawn new threads at interpreter shutdown). In that case self._running
-        will be False after running this function.
+        spawn new threads at interpreter shutdown). In that case
+        `__shutdown_requested` will be set after running this function.
         """
         if self._thread_for_pid == os.getpid() and self._thread is not None:
             return None