diff --git a/src/corelib/text/qlocale.cpp b/src/corelib/text/qlocale.cpp
index d15bdd93cd..c7ecddb484 100644
--- a/src/corelib/text/qlocale.cpp
+++ b/src/corelib/text/qlocale.cpp
@@ -788,25 +788,42 @@ static void updateSystemPrivate()
 }
 #endif // !QT_NO_SYSTEMLOCALE
 
-static const QLocaleData *systemData()
+static const QLocaleData *systemData(qsizetype *sysIndex = nullptr)
 {
 #ifndef QT_NO_SYSTEMLOCALE
     /*
       Copy over the information from the fallback locale and modify.
 
-      This modifies (cross-thread) global state, so take care to only call it in
-      one thread.
+      If sysIndex is passed, it should be the m_index of the system locale's
+      QLocalePrivate, which we'll update if it needs it.
+
+      This modifies (cross-thread) global state, so is mutex-protected.
     */
     {
+        Q_CONSTINIT static QLocaleId sysId;
+        bool updated = false;
+
         Q_CONSTINIT static QBasicMutex systemDataMutex;
         systemDataMutex.lock();
-        if (systemLocaleData.m_language_id == 0)
+        if (systemLocaleData.m_language_id == 0) {
             updateSystemPrivate();
+            updated = true;
+        }
+        // Initialization of system private has *sysIndex == -1 to hit this.
+        if (sysIndex && (updated || *sysIndex < 0)) {
+            const QLocaleId nowId = systemLocaleData.id();
+            if (sysId != nowId || *sysIndex < 0) {
+                // This look-up may be expensive:
+                *sysIndex = QLocaleData::findLocaleIndex(nowId);
+                sysId = nowId;
+            }
+        }
         systemDataMutex.unlock();
     }
 
     return &systemLocaleData;
 #else
+    Q_UNUSED(sysIndex);
     return locale_data;
 #endif
 }
@@ -2749,8 +2766,19 @@ QString QLocale::toString(double f, char format, int precision) const
 
 QLocale QLocale::system()
 {
-    QT_PREPEND_NAMESPACE(systemData)(); // Ensure system data is up to date.
-    static QLocalePrivate locale(systemData(), defaultIndex(), DefaultNumberOptions, 1);
+    constexpr auto sysData = []() {
+        // Same return as systemData(), but leave the setup to the actual call to it.
+#ifdef QT_NO_SYSTEMLOCALE
+        return locale_data;
+#else
+        return &systemLocaleData;
+#endif
+    };
+    Q_CONSTINIT static QLocalePrivate locale(sysData(), -1, DefaultNumberOptions, 1);
+    // Calling systemData() ensures system data is up to date; we also need it
+    // to ensure that locale's index stays up to date:
+    systemData(&locale.m_index);
+    Q_ASSERT(locale.m_index >= 0 && locale.m_index < locale_data_size);
 
     return QLocale(locale);
 }
diff --git a/src/corelib/text/qlocale_p.h b/src/corelib/text/qlocale_p.h
index 6c1826c846..4dcfb0329b 100644
--- a/src/corelib/text/qlocale_p.h
+++ b/src/corelib/text/qlocale_p.h
@@ -532,7 +532,7 @@ public:
     // System locale has an m_data all its own; all others have m_data = locale_data + m_index
     const QLocaleData *const m_data;
     QBasicAtomicInt ref;
-    const qsizetype m_index;
+    qsizetype m_index; // System locale needs this updated when m_data->id() changes.
     QLocale::NumberOptions m_numberOptions;
 
     static QBasicAtomicInt s_generation;
diff --git a/tests/auto/corelib/text/qlocale/syslocaleapp/syslocaleapp.cpp b/tests/auto/corelib/text/qlocale/syslocaleapp/syslocaleapp.cpp
index 1194cf0895..7cfdf161ea 100644
--- a/tests/auto/corelib/text/qlocale/syslocaleapp/syslocaleapp.cpp
+++ b/tests/auto/corelib/text/qlocale/syslocaleapp/syslocaleapp.cpp
@@ -1,15 +1,21 @@
 // Copyright (C) 2016 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
 #include <QLocale>
+#include <QCalendar>
 #include <QCoreApplication>
 #include <QTextStream>
 
 int main(int argc, char** argv)
 {
     QCoreApplication app(argc, argv);
-    QLocale l;
+    // Setting a default locale should not mess up the system one.
+    QLocale::setDefault(QLocale::Persian);
+    QLocale l = QLocale::system();
+    // A non-Roman calendar will use CLDR data instead of system data, so needs
+    // to have got the right locale index to look that up.
+    QCalendar cal = QCalendar(QCalendar::System::Jalali);
     QTextStream str(stdout);
-    str << l.name();
+    str << l.name() << ' ' << cal.standaloneMonthName(l, 2);
 
     return 0;
 }
diff --git a/tests/auto/corelib/text/qlocale/tst_qlocale.cpp b/tests/auto/corelib/text/qlocale/tst_qlocale.cpp
index 912acfd42d..65d7d9ecbf 100644
--- a/tests/auto/corelib/text/qlocale/tst_qlocale.cpp
+++ b/tests/auto/corelib/text/qlocale/tst_qlocale.cpp
@@ -591,45 +591,49 @@ void tst_QLocale::systemLocale_data()
     // Note that the accepted values for fields are implementation-dependent;
     // the template is language[_territory][.codeset][@modifier]
 
+    // "Ordibehesht" is the name (as adapted to English, German or Norsk) of the
+    // second month of the Jalali calendar. If you see anything in Arabic,
+    // setDefault(Persian) has interfered with the system locale setup.
+
     // Vanilla:
-    ADD_CTOR_TEST("C", "C");
+    ADD_CTOR_TEST("C", "C Ordibehesht");
 
     // Standard forms:
-    ADD_CTOR_TEST("en", "en_US");
-    ADD_CTOR_TEST("en_GB", "en_GB");
-    ADD_CTOR_TEST("de", "de_DE");
+    ADD_CTOR_TEST("en", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en_GB", "en_GB Ordibehesht");
+    ADD_CTOR_TEST("de", "de_DE Ordibehescht");
     // Norsk has some quirks:
-    ADD_CTOR_TEST("no", "nb_NO");
-    ADD_CTOR_TEST("nb", "nb_NO");
-    ADD_CTOR_TEST("nn", "nn_NO");
-    ADD_CTOR_TEST("no_NO", "nb_NO");
-    ADD_CTOR_TEST("nb_NO", "nb_NO");
-    ADD_CTOR_TEST("nn_NO", "nn_NO");
+    ADD_CTOR_TEST("no", "nb_NO ordibehesht");
+    ADD_CTOR_TEST("nb", "nb_NO ordibehesht");
+    ADD_CTOR_TEST("nn", "nn_NO ordibehesht");
+    ADD_CTOR_TEST("no_NO", "nb_NO ordibehesht");
+    ADD_CTOR_TEST("nb_NO", "nb_NO ordibehesht");
+    ADD_CTOR_TEST("nn_NO", "nn_NO ordibehesht");
 
     // Not too fussy about case:
-    ADD_CTOR_TEST("DE", "de_DE");
-    ADD_CTOR_TEST("EN", "en_US");
+    ADD_CTOR_TEST("DE", "de_DE Ordibehescht");
+    ADD_CTOR_TEST("EN", "en_US Ordibehesht");
 
     // Invalid fields
-    ADD_CTOR_TEST("bla", "C");
-    ADD_CTOR_TEST("zz", "C");
-    ADD_CTOR_TEST("zz_zz", "C");
-    ADD_CTOR_TEST("zz...", "C");
-    ADD_CTOR_TEST("en.bla", "en_US");
-    ADD_CTOR_TEST("en@bla", "en_US");
-    ADD_CTOR_TEST("en_blaaa", "en_US");
-    ADD_CTOR_TEST("en_zz", "en_US");
-    ADD_CTOR_TEST("en_GB.bla", "en_GB");
-    ADD_CTOR_TEST("en_GB@.bla", "en_GB");
-    ADD_CTOR_TEST("en_GB@bla", "en_GB");
+    ADD_CTOR_TEST("bla", "C Ordibehesht");
+    ADD_CTOR_TEST("zz", "C Ordibehesht");
+    ADD_CTOR_TEST("zz_zz", "C Ordibehesht");
+    ADD_CTOR_TEST("zz...", "C Ordibehesht");
+    ADD_CTOR_TEST("en.bla", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en@bla", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en_blaaa", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en_zz", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en_GB.bla", "en_GB Ordibehesht");
+    ADD_CTOR_TEST("en_GB@.bla", "en_GB Ordibehesht");
+    ADD_CTOR_TEST("en_GB@bla", "en_GB Ordibehesht");
 
     // Empty optional fields, but with punctuators supplied
-    ADD_CTOR_TEST("en.", "en_US");
-    ADD_CTOR_TEST("en@", "en_US");
-    ADD_CTOR_TEST("en.@", "en_US");
-    ADD_CTOR_TEST("en_", "en_US");
-    ADD_CTOR_TEST("en_.", "en_US");
-    ADD_CTOR_TEST("en_.@", "en_US");
+    ADD_CTOR_TEST("en.", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en@", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en.@", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en_", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en_.", "en_US Ordibehesht");
+    ADD_CTOR_TEST("en_.@", "en_US Ordibehesht");
 #undef ADD_CTOR_TEST
 
 #if QT_CONFIG(process) // for runSysApp
@@ -638,7 +642,7 @@ void tst_QLocale::systemLocale_data()
     QString errorMessage;
     if (runSysApp(m_sysapp, QStringList(), cleanEnv, &defaultLoc, &errorMessage)) {
 #if defined(Q_OS_MACOS)
-        QString localeForInvalidLocale = "C";
+        QString localeForInvalidLocale = "C Ordibehesht";
 #else
         QString localeForInvalidLocale = defaultLoc;
 #endif