diff --git a/tools/upgradetool/src/main/java/org/openhab/core/tools/UpgradeTool.java b/tools/upgradetool/src/main/java/org/openhab/core/tools/UpgradeTool.java
index 67fc2392a..ae9479539 100644
--- a/tools/upgradetool/src/main/java/org/openhab/core/tools/UpgradeTool.java
+++ b/tools/upgradetool/src/main/java/org/openhab/core/tools/UpgradeTool.java
@@ -54,7 +54,8 @@ public class UpgradeTool {
             new ItemUnitToMetadataUpgrader(), //
             new JSProfileUpgrader(), //
             new ScriptProfileUpgrader(), //
-            new YamlConfigurationV1TagsUpgrader() // Added in 5.0
+            new YamlConfigurationV1TagsUpgrader(), // Added in 5.0
+            new HomeAssistantAddonUpgrader() // Added in 5.1
     );
 
     private static final Logger LOGGER = LoggerFactory.getLogger(UpgradeTool.class);
diff --git a/tools/upgradetool/src/main/java/org/openhab/core/tools/internal/HomeAssistantAddonUpgrader.java b/tools/upgradetool/src/main/java/org/openhab/core/tools/internal/HomeAssistantAddonUpgrader.java
new file mode 100644
index 000000000..2dee31f14
--- /dev/null
+++ b/tools/upgradetool/src/main/java/org/openhab/core/tools/internal/HomeAssistantAddonUpgrader.java
@@ -0,0 +1,127 @@
+/*
+ * Copyright (c) 2010-2025 Contributors to the openHAB project
+ *
+ * See the NOTICE file(s) distributed with this work for additional
+ * information.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ */
+package org.openhab.core.tools.internal;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+import java.util.Objects;
+
+import org.eclipse.jdt.annotation.NonNullByDefault;
+import org.eclipse.jdt.annotation.Nullable;
+import org.openhab.core.storage.json.internal.JsonStorage;
+import org.openhab.core.thing.dto.ThingDTO;
+import org.openhab.core.tools.Upgrader;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The {@link HomeAssistantAddonUpgrader} checks if the MQTT addon was previously
+ * installed, and if Home Assistant things exist, and if so installs the
+ * Home Assistant addon.
+ *
+ * @author Cody Cutrer - Initial contribution
+ * @author Jimmy Tanagra - Refactored into a separate class
+ */
+@NonNullByDefault
+public class HomeAssistantAddonUpgrader implements Upgrader {
+    private final Logger logger = LoggerFactory.getLogger(HomeAssistantAddonUpgrader.class);
+
+    @Override
+    public String getName() {
+        return "homeAssistantAddonUpgrader";
+    }
+
+    @Override
+    public String getDescription() {
+        return "Installs the Home Assistant addon if Home Assistant Things are present";
+    }
+
+    @Override
+    public boolean execute(@Nullable Path userdataPath, @Nullable Path confPath) {
+        if (userdataPath == null) {
+            logger.error("{} skipped: no userdata directory found.", getName());
+            return false;
+        }
+
+        Path addonsConfigPath = userdataPath.resolve("config/org/openhab/addons.config");
+        if (!Files.isWritable(addonsConfigPath)) {
+            logger.error("Cannot access addon config '{}', check path and access rights.", addonsConfigPath);
+            return false;
+        }
+
+        List<String> configLines;
+        try {
+            configLines = Files.readAllLines(addonsConfigPath);
+        } catch (IOException e) {
+            logger.error("Failed to read addon config '{}', check path and access rights.", addonsConfigPath);
+            return false;
+        }
+
+        boolean changed = false;
+
+        for (int i = 0; i < configLines.size(); i++) {
+            String line = Objects.requireNonNull(configLines.get(i));
+            if (line.contains(",homeassistant")) {
+                logger.info("Home Assistant addon already installed, skipping installation.");
+                return true;
+            }
+            if (line.contains(",mqtt")) {
+                String newLine = line.replace(",mqtt", ",mqtt,homeassistant");
+                configLines.set(i, newLine);
+                changed = true;
+                break;
+            }
+        }
+        if (!changed) {
+            logger.info("MQTT addon not installed, skipping Home Assistant addon installation.");
+            return true;
+        }
+
+        Path thingJsonDatabasePath = userdataPath.resolve("jsondb/org.openhab.core.thing.Thing.json");
+        if (!Files.isReadable(thingJsonDatabasePath)) {
+            logger.error("Cannot access thing database '{}', check path and access rights.", thingJsonDatabasePath);
+            return false;
+        }
+
+        JsonStorage<ThingDTO> thingStorage = new JsonStorage<>(thingJsonDatabasePath.toFile(), null, 5, 0, 0,
+                List.of());
+
+        boolean hasHAThing = false;
+        for (String thingId : thingStorage.getKeys()) {
+            ThingDTO thing = thingStorage.get(thingId);
+            if (thing != null && thing.thingTypeUID != null
+                    && thing.thingTypeUID.toString().startsWith("mqtt:homeassistant")) {
+                hasHAThing = true;
+                break;
+            }
+        }
+
+        if (!hasHAThing) {
+            logger.info("No Home Assistant Things found, skipping addon installation.");
+            return true;
+        }
+
+        logger.info("Marking Home Assistant addon for installation.");
+
+        try {
+            Files.write(addonsConfigPath, configLines);
+        } catch (IOException e) {
+            logger.error("Failed to write addon config '{}', check path and access rights.", addonsConfigPath);
+            return false;
+        }
+
+        return true;
+    }
+}