diff --git a/src/widgets/dialogs/qmessagebox.cpp b/src/widgets/dialogs/qmessagebox.cpp
index bf56b17f55f..f24cd905ccf 100644
--- a/src/widgets/dialogs/qmessagebox.cpp
+++ b/src/widgets/dialogs/qmessagebox.cpp
@@ -2158,7 +2158,11 @@ int QMessageBoxPrivate::showOldMessageBox(QWidget *parent, QMessageBox::Icon ico
     messageBox.setDefaultButton(static_cast<QPushButton *>(buttonList.value(defaultButtonNumber)));
     messageBox.setEscapeButton(buttonList.value(escapeButtonNumber));
 
-    return messageBox.exec();
+    messageBox.exec();
+
+    // Ignore exec return value and use button index instead,
+    // as that's what the documentation promises.
+    return buttonList.indexOf(messageBox.clickedButton());
 }
 
 void QMessageBoxPrivate::retranslateStrings()
diff --git a/tests/auto/widgets/dialogs/qmessagebox/tst_qmessagebox.cpp b/tests/auto/widgets/dialogs/qmessagebox/tst_qmessagebox.cpp
index 94afff6e408..26a62451643 100644
--- a/tests/auto/widgets/dialogs/qmessagebox/tst_qmessagebox.cpp
+++ b/tests/auto/widgets/dialogs/qmessagebox/tst_qmessagebox.cpp
@@ -63,6 +63,7 @@ private slots:
     void hideNativeByDestruction();
 
     void explicitDoneAfterButtonClicked();
+    void legacyApiReturnValue();
 
     void cleanup();
 };
@@ -511,7 +512,7 @@ QT_WARNING_DISABLE_DEPRECATED
     // the button text versions
     closeHelper.start(Qt::Key_Enter);
     ret = QMessageBox::information(nullptr, "title", "text", "Yes", "No", QString(), 1);
-    COMPARE(ret, 3); // Custom button opaque result
+    COMPARE(ret, 1);
     QVERIFY(closeHelper.done());
 #endif // QT_DEPRECATED_SINCE(6, 2)
 #undef COMPARE
@@ -863,5 +864,21 @@ void tst_QMessageBox::explicitDoneAfterButtonClicked()
     QCOMPARE(rejectedSpy.size(), 3);
 }
 
+void tst_QMessageBox::legacyApiReturnValue()
+{
+    ExecCloseHelper closeHelper;
+    for (int i = 0; i < 3; ++i) {
+        closeHelper.start(Qt::Key_Enter);
+QT_WARNING_PUSH
+QT_WARNING_DISABLE_DEPRECATED
+        QCOMPARE(QMessageBox::warning(nullptr, "Title", "Text",
+            "Button 0", "Button 1", "Button 2", i), i);
+        closeHelper.start(Qt::Key_Escape);
+        QCOMPARE(QMessageBox::warning(nullptr, "Title", "Text",
+            "Button 0", "Button 1", "Button 2", 0, i), i);
+QT_WARNING_POP
+    }
+}
+
 QTEST_MAIN(tst_QMessageBox)
 #include "tst_qmessagebox.moc"