diff --git a/newsfragments/3409.bugfix.rst b/newsfragments/3409.bugfix.rst
new file mode 100644
index 000000000..09f70893c
--- /dev/null
+++ b/newsfragments/3409.bugfix.rst
@@ -0,0 +1 @@
+Fixed incorrect error message in ``run_process``: the ``stdout`` pipe check now correctly says "stdout" instead of "stdin".
diff --git a/src/trio/_subprocess.py b/src/trio/_subprocess.py
index 145c2de9b..d73ba3dc2 100644
--- a/src/trio/_subprocess.py
+++ b/src/trio/_subprocess.py
@@ -675,7 +675,7 @@ async def my_deliver_cancel(process):
     if task_status is trio.TASK_STATUS_IGNORED:
         if stdin is subprocess.PIPE:
             raise ValueError(
-                "stdout=subprocess.PIPE is only valid with nursery.start, "
+                "stdin=subprocess.PIPE is only valid with nursery.start, "
                 "since that's the only way to access the pipe; use nursery.start "
                 "or pass the data you want to write directly",
             )
diff --git a/src/trio/_tests/test_subprocess.py b/src/trio/_tests/test_subprocess.py
index 05ac69d3f..a1a70e575 100644
--- a/src/trio/_tests/test_subprocess.py
+++ b/src/trio/_tests/test_subprocess.py
@@ -367,9 +367,10 @@ async def test_run() -> None:
     with pytest.raises(UnicodeError):
         await run_process(CAT, stdin="oh no, it's text")
 
-    pipe_stdout_error = r"^stdout=subprocess\.PIPE is only valid with nursery\.start, since that's the only way to access the pipe(; use nursery\.start or pass the data you want to write directly)*$"
-    with pytest.raises(ValueError, match=pipe_stdout_error):
+    pipe_stdin_error = r"^stdin=subprocess\.PIPE is only valid with nursery\.start, since that's the only way to access the pipe; use nursery\.start or pass the data you want to write directly$"
+    with pytest.raises(ValueError, match=pipe_stdin_error):
         await run_process(CAT, stdin=subprocess.PIPE)
+    pipe_stdout_error = r"^stdout=subprocess\.PIPE is only valid with nursery\.start, since that's the only way to access the pipe$"
     with pytest.raises(ValueError, match=pipe_stdout_error):
         await run_process(CAT, stdout=subprocess.PIPE)
     with pytest.raises(