diff --git a/src/gui/Application.cpp b/src/gui/Application.cpp
index 1f9543f26..4b3b488c7 100644
--- a/src/gui/Application.cpp
+++ b/src/gui/Application.cpp
@@ -307,13 +307,26 @@ void Application::socketReadyRead()
     }
 
     QStringList fileNames;
-    in >> fileNames;
-    for (const QString& fileName : asConst(fileNames)) {
-        const QFileInfo fInfo(fileName);
-        if (fInfo.isFile() && fInfo.suffix().toLower() == "kdbx") {
-            emit openFile(fileName);
+    quint32 id;
+    in >> id;
+
+    // TODO: move constants to enum
+    switch (id) {
+    case 1:
+        in >> fileNames;
+        for (const QString& fileName : asConst(fileNames)) {
+            const QFileInfo fInfo(fileName);
+            if (fInfo.isFile() && fInfo.suffix().toLower() == "kdbx") {
+                emit openFile(fileName);
+            }
         }
+
+        break;
+    case 2:
+        getMainWindow()->lockAllDatabases();
+        break;
     }
+
     socket->deleteLater();
 }
 
@@ -326,6 +339,12 @@ bool Application::isAlreadyRunning() const
     return config()->get(Config::SingleInstance).toBool() && m_alreadyRunning;
 }
 
+/**
+ * Send to-open file names to the running UI instance
+ *
+ * @param fileNames - list of file names to open
+ * @return true if all operations succeeded (connection made, data sent, connection closed)
+ */
 bool Application::sendFileNamesToRunningInstance(const QStringList& fileNames)
 {
     QLocalSocket client;
@@ -338,13 +357,48 @@ bool Application::sendFileNamesToRunningInstance(const QStringList& fileNames)
     QByteArray data;
     QDataStream out(&data, QIODevice::WriteOnly);
     out.setVersion(QDataStream::Qt_5_0);
-    out << quint32(0) << fileNames;
+    out << quint32(0); // reserve space for block size
+    out << quint32(1); // ID for file name send. TODO: move to enum
+    out << fileNames; // send file names to be opened
     out.device()->seek(0);
-    out << quint32(data.size() - sizeof(quint32));
+    out << quint32(data.size() - sizeof(quint32)); // replace the previous constant 0 with block size
 
     const bool writeOk = client.write(data) != -1 && client.waitForBytesWritten(WaitTimeoutMSec);
     client.disconnectFromServer();
-    const bool disconnected = client.waitForDisconnected(WaitTimeoutMSec);
+    const bool disconnected =
+        client.state() == QLocalSocket::UnconnectedState || client.waitForDisconnected(WaitTimeoutMSec);
+    return writeOk && disconnected;
+}
+
+/**
+ * Locks all open databases in the running instance
+ *
+ * @return true if the "please lock" signal was sent successfully
+ */
+bool Application::sendLockToInstance()
+{
+    // Make a connection to avoid SIGSEGV
+    QLocalSocket client;
+    client.connectToServer(m_socketName);
+    const bool connected = client.waitForConnected(WaitTimeoutMSec);
+    if (!connected) {
+        return false;
+    }
+
+    // Send lock signal
+    QByteArray data;
+    QDataStream out(&data, QIODevice::WriteOnly);
+    out.setVersion(QDataStream::Qt_5_0);
+    out << quint32(0); // reserve space for block size
+    out << quint32(2); // ID for database lock. TODO: move to enum
+    out.device()->seek(0);
+    out << quint32(data.size() - sizeof(quint32)); // replace the previous constant 0 with block size
+
+    // Finish gracefully
+    const bool writeOk = client.write(data) != -1 && client.waitForBytesWritten(WaitTimeoutMSec);
+    client.disconnectFromServer();
+    const bool disconnected =
+        client.state() == QLocalSocket::UnconnectedState || client.waitForConnected(WaitTimeoutMSec);
     return writeOk && disconnected;
 }
 
diff --git a/src/gui/Application.h b/src/gui/Application.h
index 5f79f64b2..f71ee86ae 100644
--- a/src/gui/Application.h
+++ b/src/gui/Application.h
@@ -50,6 +50,7 @@ public:
     bool isDarkTheme() const;
 
     bool sendFileNamesToRunningInstance(const QStringList& fileNames);
+    bool sendLockToInstance();
 
     void restart();
 
diff --git a/src/main.cpp b/src/main.cpp
index 5f175173e..1dba5f426 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -71,6 +71,7 @@ int main(int argc, char** argv)
     QCommandLineOption configOption("config", QObject::tr("path to a custom config file"), "config");
     QCommandLineOption localConfigOption(
         "localconfig", QObject::tr("path to a custom local config file"), "localconfig");
+    QCommandLineOption lockOption("lock", QObject::tr("lock all open databases"));
     QCommandLineOption keyfileOption("keyfile", QObject::tr("key file of the database"), "keyfile");
     QCommandLineOption pwstdinOption("pw-stdin", QObject::tr("read password of the database from stdin"));
     QCommandLineOption allowScreenCaptureOption("allow-screencapture",
@@ -81,6 +82,7 @@ int main(int argc, char** argv)
     QCommandLineOption debugInfoOption(QStringList() << "debug-info", QObject::tr("Displays debugging information."));
     parser.addOption(configOption);
     parser.addOption(localConfigOption);
+    parser.addOption(lockOption);
     parser.addOption(keyfileOption);
     parser.addOption(pwstdinOption);
     parser.addOption(debugInfoOption);
@@ -110,12 +112,23 @@ int main(int argc, char** argv)
     }
 
     // Process single instance and early exit if already running
+    // FIXME: this is a *mess* and it is entirely my fault. --wundrweapon
     const QStringList fileNames = parser.positionalArguments();
     if (app.isAlreadyRunning()) {
-        if (!fileNames.isEmpty()) {
-            app.sendFileNamesToRunningInstance(fileNames);
+        if (parser.isSet(lockOption)) {
+            if (app.sendLockToInstance()) {
+                qInfo() << QObject::tr("Locked databases.").toUtf8().constData();
+            } else {
+                qWarning() << QObject::tr("Database failed to lock.").toUtf8().constData();
+                return EXIT_FAILURE;
+            }
+        } else {
+            if (!fileNames.isEmpty()) {
+                app.sendFileNamesToRunningInstance(fileNames);
+            }
+
+            qWarning() << QObject::tr("Another instance of KeePassXC is already running.").toUtf8().constData();
         }
-        qWarning() << QObject::tr("Another instance of KeePassXC is already running.").toUtf8().constData();
         return EXIT_SUCCESS;
     }