diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 7465b8a..0071eb1 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '3.2', '3.1', '3.0', '2.7', head, jruby, truffleruby-head ]
+        ruby: [ '3.2', '3.1', '3.0', '2.7', head, jruby, truffleruby, truffleruby-head ]
         os: [ ubuntu-latest, macos-latest ]
     runs-on: ${{ matrix.os }}
     steps:
diff --git a/lib/timeout.rb b/lib/timeout.rb
index 7f40baf..1d092f7 100644
--- a/lib/timeout.rb
+++ b/lib/timeout.rb
@@ -120,7 +120,7 @@ def self.create_timeout_thread
         requests.reject!(&:done?)
       end
     end
-    ThreadGroup::Default.add(watcher)
+    ThreadGroup::Default.add(watcher) unless watcher.group.enclosed?
     watcher.name = "Timeout stdlib thread"
     watcher.thread_variable_set(:"\0__detached_thread__", true)
     watcher
diff --git a/test/test_timeout.rb b/test/test_timeout.rb
index 2d3dd16..c3349d0 100644
--- a/test/test_timeout.rb
+++ b/test/test_timeout.rb
@@ -172,4 +172,17 @@ def test_threadgroup
     end;
   end
 
+  # https://github.com/ruby/timeout/issues/24
+  def test_handling_enclosed_threadgroup
+    assert_separately(%w[-rtimeout], <<-'end;')
+      Thread.new {
+        t = Thread.current
+        group = ThreadGroup.new
+        group.add(t)
+        group.enclose
+
+        assert_equal 42, Timeout.timeout(1) { 42 }
+      }.join
+    end;
+  end
 end