diff --git a/can/thread_safe_bus.py b/can/thread_safe_bus.py
index 518604364..8b3db4d8f 100644
--- a/can/thread_safe_bus.py
+++ b/can/thread_safe_bus.py
@@ -1,4 +1,3 @@
-from contextlib import nullcontext
 from threading import RLock
 from typing import TYPE_CHECKING, Any, cast
 
@@ -29,6 +28,7 @@ class ThreadSafeBus(
     wraps around an existing interface instance. All public methods
     of that base class are now safe to be called from multiple threads.
     The send and receive methods are synchronized separately.
+    Default thread-based periodic sends share the send lock.
 
     Use this as a drop-in replacement for :class:`~can.BusABC`.
 
@@ -67,14 +67,14 @@ def __init__(
             "BusABC", object.__getattribute__(self, "__wrapped__")
         )
 
-        # now, BusABC.send_periodic() does not need a lock anymore, but the
-        # implementation still requires a context manager
-        self._self_wrapped._lock_send_periodic = nullcontext()  # type: ignore[assignment]
-
         # init locks for sending and receiving separately
         self._self_lock_send = RLock()
         self._self_lock_recv = RLock()
 
+        # Default periodic tasks call the wrapped backend directly, so their
+        # worker threads must share the lock used by explicit sends.
+        self._self_wrapped._lock_send_periodic = self._self_lock_send  # type: ignore[assignment]
+
     def recv(self, timeout: float | None = None) -> Message | None:
         with self._self_lock_recv:
             return self._self_wrapped.recv(timeout=timeout)
diff --git a/doc/changelog.d/1620.fixed.rst b/doc/changelog.d/1620.fixed.rst
new file mode 100644
index 000000000..1bf5c8dbe
--- /dev/null
+++ b/doc/changelog.d/1620.fixed.rst
@@ -0,0 +1,2 @@
+Synchronize default thread-based periodic sends with explicit sends on
+``ThreadSafeBus`` to prevent concurrent calls into the underlying backend.
diff --git a/test/test_thread_safe_periodic.py b/test/test_thread_safe_periodic.py
new file mode 100644
index 000000000..4cddddaa8
--- /dev/null
+++ b/test/test_thread_safe_periodic.py
@@ -0,0 +1,125 @@
+"""Behavioral regressions for default periodic sends on ThreadSafeBus."""
+
+import threading
+import unittest
+from unittest.mock import patch
+
+import can
+
+
+class ThreadSafePeriodicTest(unittest.TestCase):
+    def check_serialized_sends(self, first_is_periodic):
+        first_entered = threading.Event()
+        release_first = threading.Event()
+        contender_reached = threading.Event()
+        second_entered = threading.Event()
+        counter_lock = threading.Lock()
+        contender_id = None
+        active = 0
+        maximum_active = 0
+        errors = []
+
+        class ObservedRLock:
+            """Keep real RLock semantics while exposing an attempted acquisition."""
+
+            def __init__(self):
+                self.lock = threading.RLock()
+
+            def __enter__(self):
+                if threading.get_ident() == contender_id:
+                    contender_reached.set()
+                self.lock.acquire()
+                return self
+
+            def __exit__(self, *args):
+                self.lock.release()
+
+        def mark_contender(message):
+            nonlocal contender_id
+            contender_id = threading.get_ident()
+
+        first = can.Message(arbitration_id=0x100, data=[1], is_extended_id=False)
+        second = can.Message(arbitration_id=0x200, data=[2], is_extended_id=False)
+        tasks = []
+        manual_thread = None
+
+        with patch("can.thread_safe_bus.RLock", ObservedRLock):
+            bus = can.ThreadSafeBus(
+                interface="virtual", channel=self.id(), receive_own_messages=True
+            )
+        backend = bus.__wrapped__
+        original_send = backend.send
+
+        def observed_send(msg, timeout=None):
+            nonlocal active, maximum_active
+            with counter_lock:
+                active += 1
+                maximum_active = max(maximum_active, active)
+            try:
+                if msg.arbitration_id == first.arbitration_id:
+                    first_entered.set()
+                    if not release_first.wait(5):
+                        raise TimeoutError("test did not release the first send")
+                else:
+                    # Unlocked implementations reach the backend; locked ones
+                    # signal at attempted acquisition before entering here.
+                    contender_reached.set()
+                    second_entered.set()
+                return original_send(msg, timeout=timeout)
+            except BaseException as exc:
+                errors.append(exc)
+                raise
+            finally:
+                with counter_lock:
+                    active -= 1
+
+        def manual_send():
+            try:
+                bus.send(first)
+            except BaseException as exc:
+                errors.append(exc)
+
+        try:
+            with patch.object(backend, "send", side_effect=observed_send):
+                try:
+                    if first_is_periodic:
+                        tasks.append(bus.send_periodic(first, 0.01))
+                    else:
+                        manual_thread = threading.Thread(target=manual_send)
+                        manual_thread.start()
+                    self.assertTrue(first_entered.wait(5))
+                    tasks.append(
+                        bus.send_periodic(
+                            second, 0.01, modifier_callback=mark_contender
+                        )
+                    )
+                    # A positive barrier works whether the contender reaches
+                    # the shared lock or incorrectly enters the backend.
+                    self.assertTrue(contender_reached.wait(5))
+                    release_first.set()
+                    self.assertTrue(second_entered.wait(5))
+                finally:
+                    release_first.set()
+                    for task in tasks:
+                        task.stop()
+                    if manual_thread is not None:
+                        manual_thread.join(5)
+                        self.assertFalse(manual_thread.is_alive())
+                    for task in tasks:
+                        task.thread.join(5)
+                        self.assertFalse(task.thread.is_alive())
+
+            self.assertEqual(errors, [])
+            self.assertEqual(maximum_active, 1)
+            received = []
+            while (message := bus.recv(0)) is not None:
+                received.append((message.arbitration_id, bytes(message.data)))
+            self.assertEqual(set(received), {(0x100, b"\x01"), (0x200, b"\x02")})
+        finally:
+            bus.shutdown()
+
+    def test_manual_and_periodic_send_do_not_overlap(self):
+        self.check_serialized_sends(first_is_periodic=False)
+
+    def test_multiple_periodic_sends_do_not_overlap(self):
+        self.check_serialized_sends(first_is_periodic=True)