diff --git a/src/corelib/text/qlocale.cpp b/src/corelib/text/qlocale.cpp
index 6d6c01da662..d3dfff1497f 100644
--- a/src/corelib/text/qlocale.cpp
+++ b/src/corelib/text/qlocale.cpp
@@ -4750,6 +4750,11 @@ QStringList QLocale::uiLanguages(TagSeparator separator) const
     const bool isSystem = d->m_data == &systemLocaleData;
     if (isSystem) {
         uiLanguages = systemLocale()->query(QSystemLocale::UILanguages).toStringList();
+        if (separator != TagSeparator::Dash) {
+            // Map from default separator, Dash, used by backends:
+            const QChar join = QLatin1Char(sep);
+            uiLanguages = uiLanguages.replaceInStrings(u"-", QStringView(&join, 1));
+        }
         // ... but we need to include likely-adjusted forms of each of those, too.
         // For now, collect up locale Ids representing the entries, for later processing:
         for (const auto &entry : std::as_const(uiLanguages))
diff --git a/tests/auto/corelib/text/qlocale/tst_qlocale.cpp b/tests/auto/corelib/text/qlocale/tst_qlocale.cpp
index 5551f6c4a97..05660684e75 100644
--- a/tests/auto/corelib/text/qlocale/tst_qlocale.cpp
+++ b/tests/auto/corelib/text/qlocale/tst_qlocale.cpp
@@ -4102,6 +4102,8 @@ void tst_QLocale::mySystemLocale()
             qDebug("\n\t%s", qPrintable(QLocale::system().uiLanguages().join(u"\n\t")));
         });
         QCOMPARE(QLocale::system().uiLanguages(), uiLanguages);
+        QCOMPARE(QLocale::system().uiLanguages(QLocale::TagSeparator::Underscore),
+                 uiLanguages.replaceInStrings(u"-", u"_"));
         reporter.dismiss();
     }