diff --git a/packages/lib/plugin.js b/packages/lib/plugin.js
index 4dd71d08..834d2c96 100644
--- a/packages/lib/plugin.js
+++ b/packages/lib/plugin.js
@@ -256,6 +256,7 @@ class BaseMasterPlugin {
 	 * or received notice from a slave that the status of an instance has
 	 * changed.  The possible statuses that can be notified about are:
 	 * - `unassigned:`: Instance is no longer asssigned to a slave.
+	 * - `unknown`: Slave assigned to instance is offline.
 	 * - `stopped`: Instance is no longer running or was just assigned to a
 	 *   slave.
 	 * - `starting`: Instance is in the process of starting up.
@@ -270,7 +271,8 @@ class BaseMasterPlugin {
 	 * they are assigned to a slave, when the slave then connects to the
 	 * master and informs of the current status of its instances this hook
 	 * is invoked for all those instances.  You can detect this situation by
-	 * checking if prev equals `unknown`.
+	 * checking if prev equals `unknown`.  If the slave disconnects the
+	 * instances will again get the `unknown` status.
 	 *
 	 * When instances are created on the master they will notify of a status
 	 * change with prev set to null.  While the status of new instances is
diff --git a/packages/master/src/SlaveConnection.js b/packages/master/src/SlaveConnection.js
index f4c71d1b..6c2319dd 100644
--- a/packages/master/src/SlaveConnection.js
+++ b/packages/master/src/SlaveConnection.js
@@ -39,6 +39,20 @@ class SlaveConnection extends BaseConnection {
 				}
 			});
 		}
+
+		this.connector.on("close", () => {
+			// Update status to unknown for instances on this slave.
+			for (let instance of this._master.instances.values()) {
+				if (instance.config.get("instance.assigned_slave") !== this._id) {
+					continue;
+				}
+
+				let prev = instance.status;
+				instance.status = "unknown";
+				this._master.instanceUpdated(instance);
+				libPlugin.invokeHook(this._master.plugins, "onInstanceStatusChanged", instance, prev);
+			}
+		});
 	}
 
 	/**
diff --git a/test/integration/clusterio.js b/test/integration/clusterio.js
index c5007d57..ea92fd4d 100644
--- a/test/integration/clusterio.js
+++ b/test/integration/clusterio.js
@@ -88,6 +88,9 @@ describe("Integration of Clusterio", function() {
 					slaveProcess = await spawn(
 						"alt-slave:", `node ../../packages/slave run --config ${config}`, /Started slave/
 					);
+					// Add instance to test the unknown status afterwards
+					await execCtl("instance create alt-test --id 99");
+					await execCtl("instance assign alt-test 5");
 				} finally {
 					if (slaveProcess) {
 						slaveProcess.kill("SIGINT");
@@ -524,7 +527,7 @@ describe("Integration of Clusterio", function() {
 		describe("instanceUpdateEventHandler()", function() {
 			it("should have triggered for the previous instance status updates", function() {
 				let statusesToCheck = new Set([
-					"unassigned", "stopped", "creating_save", "exporting_data",
+					"unassigned", "unknown", "stopped", "creating_save", "exporting_data",
 					"starting", "running", "stopping", "deleted",
 				]);
 				let statusesNotSeen = new Set(statusesToCheck);