diff --git a/Lib/selectors.py b/Lib/selectors.py
index 598845d43b0..6d569c30adf 100644
--- a/Lib/selectors.py
+++ b/Lib/selectors.py
@@ -174,6 +174,8 @@ class BaseSelector(metaclass=ABCMeta):
         SelectorKey for this file object
         """
         mapping = self.get_map()
+        if mapping is None:
+            raise RuntimeError('Selector is closed')
         try:
             return mapping[fileobj]
         except KeyError:
@@ -256,6 +258,7 @@ class _BaseSelectorImpl(BaseSelector):
 
     def close(self):
         self._fd_to_key.clear()
+        self._map = None
 
     def get_map(self):
         return self._map
diff --git a/Lib/test/test_selectors.py b/Lib/test/test_selectors.py
index 493a5a6d50c..15f71c11831 100644
--- a/Lib/test/test_selectors.py
+++ b/Lib/test/test_selectors.py
@@ -178,14 +178,17 @@ class BaseSelectorTestCase(unittest.TestCase):
         s = self.SELECTOR()
         self.addCleanup(s.close)
 
+        mapping = s.get_map()
         rd, wr = self.make_socketpair()
 
         s.register(rd, selectors.EVENT_READ)
         s.register(wr, selectors.EVENT_WRITE)
 
         s.close()
-        self.assertRaises(KeyError, s.get_key, rd)
-        self.assertRaises(KeyError, s.get_key, wr)
+        self.assertRaises(RuntimeError, s.get_key, rd)
+        self.assertRaises(RuntimeError, s.get_key, wr)
+        self.assertRaises(KeyError, mapping.__getitem__, rd)
+        self.assertRaises(KeyError, mapping.__getitem__, wr)
 
     def test_get_key(self):
         s = self.SELECTOR()
@@ -252,8 +255,8 @@ class BaseSelectorTestCase(unittest.TestCase):
             sel.register(rd, selectors.EVENT_READ)
             sel.register(wr, selectors.EVENT_WRITE)
 
-        self.assertRaises(KeyError, s.get_key, rd)
-        self.assertRaises(KeyError, s.get_key, wr)
+        self.assertRaises(RuntimeError, s.get_key, rd)
+        self.assertRaises(RuntimeError, s.get_key, wr)
 
     def test_fileno(self):
         s = self.SELECTOR()
diff --git a/Misc/NEWS b/Misc/NEWS
index ec2e74a792f..04eb5a7aa7c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -203,6 +203,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #23209, #23225: selectors.BaseSelector.get_key() now raises a
+  RuntimeError if the selector is closed. And selectors.BaseSelector.close()
+  now clears its internal reference to the selector mapping to break a
+  reference cycle. Initial patch written by Martin Richard.
+
 - Issue #19777: Provide a home() classmethod on Path objects.  Contributed
   by Victor Salgado and Mayank Tripathi.