diff --git i/django/contrib/admin/static/admin/js/admin/RelatedObjectLookups.js w/django/contrib/admin/static/admin/js/admin/RelatedObjectLookups.js
index 5395386087..5ea8ff4017 100644
--- i/django/contrib/admin/static/admin/js/admin/RelatedObjectLookups.js
+++ w/django/contrib/admin/static/admin/js/admin/RelatedObjectLookups.js
@@ -58,9 +58,9 @@
             elem.value = chosenId;
         }
         $(elem).trigger('change');
-        const index = window.relatedWindows.indexOf(win);
+        const index = relatedWindows.indexOf(win);
         if (index > -1) {
-            window.relatedWindows.splice(index, 1);
+            relatedWindows.splice(index, 1);
         }
         win.close();
     }
@@ -211,6 +211,8 @@
     window.showAddAnotherPopup = showRelatedObjectPopup;
     window.dismissAddAnotherPopup = dismissAddRelatedObjectPopup;
 
+    window.__SECRET_INTERNALS_relatedWindows = relatedWindows;
+
     window.addEventListener('unload', function(evt) {
         window.dismissChildPopups();
     });
diff --git i/js_tests/admin/RelatedObjectLookups.test.js w/js_tests/admin/RelatedObjectLookups.test.js
index 722aa7ae7b..3d28e16163 100644
--- i/js_tests/admin/RelatedObjectLookups.test.js
+++ w/js_tests/admin/RelatedObjectLookups.test.js
@@ -8,7 +8,6 @@ QUnit.module('admin.RelatedObjectLookups', {
             <input type="text" id="test_id" name="test" />
             <input type="text" id="many_test_id" name="many_test" class="vManyToManyRawIdAdminField" />
         `);
-        window.relatedWindows = window.relatedWindows || [];
     }
 });
 
@@ -31,10 +30,10 @@ QUnit.test('dismissRelatedLookupPopup removes window from relatedWindows array',
         name: testId,
         close: function() {}
     };
-    window.relatedWindows.push(mockWin);
-    assert.equal(window.relatedWindows.indexOf(mockWin), 0, 'Window should be in relatedWindows array');
+    window.__SECRET_INTERNALS_relatedWindows.push(mockWin);
+    assert.equal(window.__SECRET_INTERNALS_relatedWindows.indexOf(mockWin), 0, 'Window should be in relatedWindows array');
     window.dismissRelatedLookupPopup(mockWin, '123');
-    assert.equal(window.relatedWindows.indexOf(mockWin), -1, 'Window should be removed from relatedWindows array');
+    assert.equal(window.__SECRET_INTERNALS_relatedWindows.indexOf(mockWin), -1, 'Window should be removed from relatedWindows array');
 });
 
 QUnit.test('dismissRelatedLookupPopup triggers change event for single value field', function(assert) {