diff --git a/launcher/Application.cpp b/launcher/Application.cpp
index 387f735c..ae7a69c6 100644
--- a/launcher/Application.cpp
+++ b/launcher/Application.cpp
@@ -150,6 +150,8 @@ namespace {
 /** This is used so that we can output to the log file in addition to the CLI. */
 void appDebugOutput(QtMsgType type, const QMessageLogContext &context, const QString &msg)
 {
+    const std::lock_guard<std::mutex> lock(APPLICATION->loggerMutex); // synchronized, QFile logFile is not thread-safe
+
     QString out = qFormatLogMessage(type, context, msg);
     out += QChar::LineFeed;
 
diff --git a/launcher/Application.h b/launcher/Application.h
index 1b3dc499..caee074d 100644
--- a/launcher/Application.h
+++ b/launcher/Application.h
@@ -45,6 +45,7 @@
 #include <QUrl>
 
 #include <BaseInstance.h>
+#include <mutex>
 
 #include "minecraft/launch/MinecraftServerTarget.h"
 
@@ -310,4 +311,5 @@ public:
     QList<QUrl> m_zipsToImport;
     QString m_instanceIdToShowWindowOf;
     std::unique_ptr<QFile> logFile;
+    std::mutex loggerMutex;
 };