diff --git a/newsfragments/3290.bugfix.rst b/newsfragments/3290.bugfix.rst
new file mode 100644
index 000000000..9d0fd68bd
--- /dev/null
+++ b/newsfragments/3290.bugfix.rst
@@ -0,0 +1,3 @@
+A failing exception ``__repr__`` no longer interrupts nursery bookkeeping while
+constructing cancellation diagnostics. Cancellation reasons use a placeholder
+when representing the exception raises an error, including ``RecursionError``.
diff --git a/src/trio/_core/_run.py b/src/trio/_core/_run.py
index ddd4ea7ee..76980c197 100644
--- a/src/trio/_core/_run.py
+++ b/src/trio/_core/_run.py
@@ -319,6 +319,15 @@ def expire(self, now: float) -> bool:
         return did_something
 
 
+def _safe_exception_repr(exc: BaseException) -> str:
+    try:
+        return repr(exc)
+    except BaseException:
+        # Formatting diagnostics must not interfere with nursery cleanup, even if
+        # a user-defined __repr__ raises KeyboardInterrupt or SystemExit.
+        return "<exception repr() failed>"
+
+
 @attrs.define
 class CancelReason:
     """Attached to a :class:`CancelScope` upon cancellation with details of the source of the
@@ -1265,7 +1274,7 @@ def _child_finished(
                 CancelReason(
                     source="nursery",
                     source_task=repr(task),
-                    reason=f"child task raised exception {outcome.error!r}",
+                    reason=f"child task raised exception {_safe_exception_repr(outcome.error)}",
                 ),
             )
         self._check_nursery_closed()
@@ -1282,7 +1291,7 @@ async def _nested_child_finished(
                 reason=CancelReason(
                     source="nursery",
                     source_task=repr(self._parent_task),
-                    reason=f"Code block inside nursery contextmanager raised exception {nested_child_exc!r}",
+                    reason=f"Code block inside nursery contextmanager raised exception {_safe_exception_repr(nested_child_exc)}",
                 ),
             )
         self._nested_child_running = False
diff --git a/src/trio/_core/_tests/test_cancelled.py b/src/trio/_core/_tests/test_cancelled.py
index 0280b6068..5af9f9ff9 100644
--- a/src/trio/_core/_tests/test_cancelled.py
+++ b/src/trio/_core/_tests/test_cancelled.py
@@ -1,5 +1,8 @@
 import pickle
 import re
+import subprocess
+import sys
+import textwrap
 from math import inf
 
 import pytest
@@ -10,6 +13,10 @@
 from trio.testing import wait_all_tasks_blocked
 
 from .test_ki import ki_self
+from .tutil import slow
+
+if sys.version_info < (3, 11):
+    from exceptiongroup import ExceptionGroup
 
 
 def test_Cancelled_init() -> None:
@@ -154,6 +161,108 @@ async def cancelled_task() -> None:
             await nursery.start(failing_task)
 
 
+@pytest.mark.parametrize("from_child", [False, True])
+@pytest.mark.parametrize("grouped", [False, True])
+@pytest.mark.parametrize(
+    "repr_error", [ValueError, RecursionError, KeyboardInterrupt, SystemExit]
+)
+async def test_nursery_exception_repr_failure(
+    from_child: bool,
+    grouped: bool,
+    repr_error: type[BaseException],
+) -> None:
+    class UnprintableError(Exception):
+        def __repr__(self) -> str:
+            raise repr_error
+
+    original = UnprintableError()
+    error: Exception = original
+    if grouped:
+        for _ in range(20):
+            error = ExceptionGroup("nested", [error])
+
+    reasons: list[str | None] = []
+    sources: list[str] = []
+    finished: list[bool] = []
+
+    async def sibling() -> None:
+        try:
+            await trio.sleep_forever()
+        except Cancelled as exc:
+            reasons.append(exc.reason)
+            sources.append(exc.source)
+            raise
+        finally:
+            finished.append(True)
+
+    async def failing_task() -> None:
+        raise error
+
+    with pytest.raises(ExceptionGroup) as excinfo:
+        async with trio.open_nursery() as nursery:
+            nursery.start_soon(sibling)
+            await wait_all_tasks_blocked()
+            if from_child:
+                nursery.start_soon(failing_task)
+            else:
+                raise error
+
+    propagated: BaseException = excinfo.value
+    while isinstance(propagated, ExceptionGroup):
+        assert len(propagated.exceptions) == 1
+        propagated = propagated.exceptions[0]
+    assert propagated is original
+    assert finished == [True]
+    assert sources == ["nursery"]
+    prefix = "child task" if from_child else "Code block inside nursery contextmanager"
+    assert reasons == [f"{prefix} raised exception <exception repr() failed>"]
+    # The nursery has finished unwinding; cancellation must not leak into its parent.
+    await trio.lowlevel.checkpoint()
+
+
+@slow
+def test_nursery_exception_repr_recursion() -> None:
+    # A real recursion overflow disables coverage tracing on PyPy. Isolate it so
+    # coverage can still measure the remaining tests in the parent process.
+    code = textwrap.dedent(
+        """
+        import sys
+        import trio
+
+        if sys.version_info < (3, 11):
+            from exceptiongroup import ExceptionGroup
+
+        class RecursiveError(Exception):
+            def __repr__(self):
+                return repr(self)
+
+        original = RecursiveError()
+
+        async def failing_task():
+            raise original
+
+        async def main():
+            try:
+                async with trio.open_nursery() as nursery:
+                    nursery.start_soon(failing_task)
+            except ExceptionGroup as exc:
+                assert len(exc.exceptions) == 1
+                assert exc.exceptions[0] is original
+            else:
+                raise AssertionError("original exception was lost")
+
+        trio.run(main)
+        """,
+    )
+    result = subprocess.run(
+        [sys.executable, "-c", code],
+        capture_output=True,
+        text=True,
+        timeout=30,
+    )
+    assert result.returncode == 0, result.stderr
+
+
 async def test_cancel_reason_not_overwritten() -> None:
     with trio.CancelScope() as cs:
         cs.cancel()