diff --git a/README.rst b/README.rst
index 14c3286..9e3d4a4 100644
--- a/README.rst
+++ b/README.rst
@@ -392,6 +392,11 @@ to 100 seconds::
 Changelog
 =========
 
+Unreleased
+----------
+
+- Block signals in timer threads so tests can control their own signal masks.
+
 2.5.0
 -----
 
diff --git a/pytest_timeout.py b/pytest_timeout.py
index 192a16c..0b7f9a0 100644
--- a/pytest_timeout.py
+++ b/pytest_timeout.py
@@ -322,7 +322,17 @@ def cancel():
             timer.join()
 
         item.cancel_timeout = cancel
-        timer.start()
+        if hasattr(signal, "pthread_sigmask"):
+            # The timer must not receive signals intended for the test.
+            previous_mask = signal.pthread_sigmask(
+                signal.SIG_BLOCK, signal.valid_signals()
+            )
+            try:
+                timer.start()
+            finally:
+                signal.pthread_sigmask(signal.SIG_SETMASK, previous_mask)
+        else:
+            timer.start()
     return True
 
 
diff --git a/test_pytest_timeout.py b/test_pytest_timeout.py
index 178cb3e..bc57dc2 100644
--- a/test_pytest_timeout.py
+++ b/test_pytest_timeout.py
@@ -1,7 +1,9 @@
 import os.path
 import signal
 import sys
+import threading
 import time
+from types import SimpleNamespace
 
 import pexpect
 import pytest
@@ -71,6 +73,83 @@ def test_foo():
     assert "++ Timeout ++" in result.stdout.lines[-1]
 
 
+@pytest.mark.skipif(
+    not hasattr(signal, "pthread_sigmask"), reason="OS has no pthread_sigmask"
+)
+def test_thread_timer_blocks_signals(monkeypatch):
+    import pytest_timeout
+
+    masks = []
+    completed = threading.Event()
+
+    def timeout_timer(item, settings):
+        masks.append(signal.pthread_sigmask(signal.SIG_BLOCK, []))
+        completed.set()
+
+    monkeypatch.setattr(pytest_timeout, "timeout_timer", timeout_timer)
+    item = SimpleNamespace(nodeid="test_thread_timer_blocks_signals")
+    settings = pytest_timeout.Settings(0, "thread", False, False)
+    original_mask = signal.pthread_sigmask(signal.SIG_BLOCK, {signal.SIGUSR1})
+    try:
+        caller_mask = signal.pthread_sigmask(signal.SIG_BLOCK, [])
+        pytest_timeout.pytest_timeout_set_timer(item, settings)
+        try:
+            assert completed.wait(5)
+        finally:
+            pytest_timeout.pytest_timeout_cancel_timer(item)
+        assert masks == [signal.valid_signals() - {signal.SIGKILL, signal.SIGSTOP}]
+        assert signal.pthread_sigmask(signal.SIG_BLOCK, []) == caller_mask
+    finally:
+        signal.pthread_sigmask(signal.SIG_SETMASK, original_mask)
+
+
+@pytest.mark.skipif(
+    not hasattr(signal, "pthread_sigmask"), reason="OS has no pthread_sigmask"
+)
+def test_thread_timer_start_error_restores_signal_mask(monkeypatch):
+    import pytest_timeout
+
+    masks = []
+
+    def start(timer):
+        masks.append(signal.pthread_sigmask(signal.SIG_BLOCK, []))
+        raise RuntimeError("Cannot start timer")
+
+    monkeypatch.setattr(threading.Timer, "start", start)
+    item = SimpleNamespace(nodeid="test_thread_timer_start_error")
+    settings = pytest_timeout.Settings(0, "thread", False, False)
+    caller_mask = signal.pthread_sigmask(signal.SIG_BLOCK, [])
+    try:
+        with pytest.raises(RuntimeError, match="Cannot start timer"):
+            pytest_timeout.pytest_timeout_set_timer(item, settings)
+        assert signal.pthread_sigmask(signal.SIG_BLOCK, []) == caller_mask
+        assert masks == [signal.valid_signals() - {signal.SIGKILL, signal.SIGSTOP}]
+    finally:
+        signal.pthread_sigmask(signal.SIG_SETMASK, caller_mask)
+
+
+def test_thread_timer_without_pthread_sigmask(monkeypatch):
+    import pytest_timeout
+
+    calls = []
+    completed = threading.Event()
+
+    def timeout_timer(item, settings):
+        calls.append(item)
+        completed.set()
+
+    monkeypatch.delattr(signal, "pthread_sigmask", raising=False)
+    monkeypatch.setattr(pytest_timeout, "timeout_timer", timeout_timer)
+    item = SimpleNamespace(nodeid="test_thread_timer_without_pthread_sigmask")
+    settings = pytest_timeout.Settings(0, "thread", False, False)
+    pytest_timeout.pytest_timeout_set_timer(item, settings)
+    try:
+        assert completed.wait(5)
+    finally:
+        pytest_timeout.pytest_timeout_cancel_timer(item)
+    assert calls == [item]
+
+
 def test_cov(pytester):
     # This test requires pytest-cov
     pytest.importorskip("pytest_cov")