python
@@ -1,13 +1,112 @@
 import unittest
+from threading import Thread, Barrier
 from itertools import (
+    accumulate,
+    chain,
+    combinations_with_replacement,
+    cycle,
+    permutations,
     tee,
+    zip_longest,
 )
 from test.support import threading_helper
 
 
 threading_helper.requires_working_threading(module=True)
 
 
+def work_iterator(it):
+    while True:
+        try:
+            next(it)
+        except StopIteration:
+            break
+
+
+class ItertoolsThreading(unittest.TestCase):
+
+    @threading_helper.reap_threads
+    def test_cycle(self):
+        number_of_threads = 6
+        number_of_iterations = 10
+        number_of_cycles = 400
+
+        barrier = Barrier(number_of_threads)
+        def work(it):
+            barrier.wait()
+            for _ in range(number_of_cycles):
+                _ = next(it)
+
+        data = (1, 2, 3, 4)
+        for it in range(number_of_iterations):
+            cycle_iterator = cycle(data)
+            worker_threads = []
+            for ii in range(number_of_threads):
+                worker_threads.append(
+                    Thread(target=work, args=[cycle_iterator]))
+
+            with threading_helper.start_threads(worker_threads):
+                pass
+
+            barrier.reset()
+
+    @threading_helper.reap_threads
+    def test_chain(self):
+        number_of_threads = 6
+        number_of_iterations = 20
+
+        barrier = Barrier(number_of_threads)
+        def work(it):
+            barrier.wait()
+            while True:
+                try:
+                    next(it)
+                except StopIteration:
+                    break
+
+        data = [(1, )] * 200
+        for it in range(number_of_iterations):
+            chain_iterator = chain(*data)
+            worker_threads = []
+            for ii in range(number_of_threads):
+                worker_threads.append(
+                    Thread(target=work, args=[chain_iterator]))
+
+            with threading_helper.start_threads(worker_threads):
+                pass
+
+            barrier.reset()
+
+    @threading_helper.reap_threads
+    def test_combinations_with_replacement(self):
+        number_of_iterations = 6
+        for _ in range(number_of_iterations):
+            it = combinations_with_replacement(tuple(range(2)), 2)
+            threading_helper.run_concurrently(work_iterator, nthreads=6, args=[it])
+
+    @threading_helper.reap_threads
+    def test_permutations(self):
+        number_of_iterations = 6
+        for _ in range(number_of_iterations):
+            it = permutations(tuple(range(2)), 2)
+            threading_helper.run_concurrently(work_iterator, nthreads=6, args=[it])
+
+    @threading_helper.reap_threads
+    def test_accumulate(self):
+        number_of_iterations = 10
+        for _ in range(number_of_iterations):
+            it = accumulate(tuple(range(40)))
+            threading_helper.run_concurrently(work_iterator, nthreads=10, args=[it])
+
+    @threading_helper.reap_threads
+    def test_zip_longest(self):
+        number_of_iterations = 10
+        for _ in range(number_of_iterations):
+            it = zip_longest(list(range(4)), list(range(8)), fillvalue=0)
+            threading_helper.run_concurrently(work_iterator, nthreads=10, args=[it])
+
+
+
 class TestTeeConcurrent(unittest.TestCase):
     # itertools.tee branches share a linked list of internal data cells.
     # Concurrent iteration must not corrupt that shared state or crash the

@@ -0,0 +1,51 @@
+import unittest
+from threading import Thread, Barrier
+from itertools import combinations, product
+from test.support import threading_helper
+
+
+threading_helper.requires_working_threading(module=True)
+
+def test_concurrent_iteration(iterator, number_of_threads):
+    barrier = Barrier(number_of_threads)
+    def iterator_worker(it):
+        barrier.wait()
+        while True:
+            try:
+                _ = next(it)
+            except StopIteration:
+                return
+
+    worker_threads = []
+    for ii in range(number_of_threads):
+        worker_threads.append(
+            Thread(target=iterator_worker, args=[iterator]))
+
+    with threading_helper.start_threads(worker_threads):
+        pass
+
+    barrier.reset()
+
+class ItertoolsThreading(unittest.TestCase):
+
+    @threading_helper.reap_threads
+    def test_combinations(self):
+        number_of_threads = 10
+        number_of_iterations = 24
+
+        for it in range(number_of_iterations):
+            iterator = combinations((1, 2, 3, 4, 5), 2)
+            test_concurrent_iteration(iterator, number_of_threads)
+
+    @threading_helper.reap_threads
+    def test_product(self):
+        number_of_threads = 10
+        number_of_iterations = 24
+
+        for it in range(number_of_iterations):
+            iterator = product((1, 2, 3, 4, 5), (10, 20, 30))
+            test_concurrent_iteration(iterator, number_of_threads)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2025-03-13-20-48-58.gh-issue-123471.cM4w4f.rst b/Misc/NEWS.d/next/Library/2025-03-13-20-48-58.gh-issue-123471.cM4w4f.rst
@@ -0,0 +1 @@
+Make concurrent iterations over :class:`itertools.cycle` safe under free-threading.
diff --git a/Misc/NEWS.d/next/Library/2025-04-22-21-00-23.gh-issue-123471.asOLA2.rst b/Misc/NEWS.d/next/Library/2025-04-22-21-00-23.gh-issue-123471.asOLA2.rst
@@ -0,0 +1 @@
+Make concurrent iterations over :class:`itertools.combinations` and :class:`itertools.product` safe under free-threading.
diff --git a/Misc/NEWS.d/next/Library/2025-06-18-19-25-32.gh-issue-123471.lx1Xbt.rst b/Misc/NEWS.d/next/Library/2025-06-18-19-25-32.gh-issue-123471.lx1Xbt.rst
@@ -0,0 +1 @@
+Make concurrent iterations over :class:`itertools.chain` safe under :term:`free threading`.
diff --git a/Misc/NEWS.d/next/Library/2026-02-03-08-50-58.gh-issue-123471.yF1Gym.rst b/Misc/NEWS.d/next/Library/2026-02-03-08-50-58.gh-issue-123471.yF1Gym.rst
@@ -0,0 +1 @@
+Make concurrent iteration over :class:`itertools.combinations_with_replacement` and :class:`itertools.permutations` safe under free-threading.
diff --git a/Misc/NEWS.d/next/Library/2026-02-04-20-30-59.gh-issue-123471.1dnPvs.rst b/Misc/NEWS.d/next/Library/2026-02-04-20-30-59.gh-issue-123471.1dnPvs.rst
@@ -0,0 +1 @@
+Make concurrent iteration over :class:`itertools.accumulate` safe under free-threading.
diff --git a/Misc/NEWS.d/next/Library/2026-03-17-19-51-05.gh-issue-123471.oY4UR5.rst b/Misc/NEWS.d/next/Library/2026-03-17-19-51-05.gh-issue-123471.oY4UR5.rst
@@ -0,0 +1 @@
+Make concurrent iteration over :class:`itertools.zip_longest` safe under free-threading.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Make concurrent iterations over :class:`itertools.cycle` safe under free-threading.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Make concurrent iteration over :class:`itertools.combinations_with_replacement` and :class:`itertools.permutations` safe under free-threading.