diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index e259dc3a8e538a..c39d98104b3bfc 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -937,6 +937,10 @@ def __init__(self, args, bufsize=-1, executable=None,
             if uid < 0:
                 raise ValueError(f"User ID cannot be negative, got {uid}")
 
+        if preexec_fn == os.setsid:  # A common unnecessary legacy use.
+            start_new_session = True
+            preexec_fn = None
+
         try:
             if p2cwrite != -1:
                 self.stdin = io.open(p2cwrite, 'wb', bufsize)