diff --git a/backend/tests/instance_config.rs b/backend/tests/instance_config.rs
index 64d7854173..882207ebfe 100644
--- a/backend/tests/instance_config.rs
+++ b/backend/tests/instance_config.rs
@@ -1583,3 +1583,49 @@ async fn declarative_sync_rejects_an_unusable_default_allowed_origins(db: Pool<P
     .await
     .expect("a valid origin list must sync");
 }
+
+/// While Windmill has databases on the external cluster, a sync may change its login but not
+/// point it at another cluster: the databases would be stranded there, and their registry and
+/// role passwords applied to a cluster that has neither.
+#[cfg(all(feature = "enterprise", feature = "private"))]
+#[sqlx::test(fixtures("base"))]
+async fn declarative_sync_keeps_the_external_cluster_while_it_holds_databases(db: Pool<Postgres>) {
+    clear_settings_and_configs(&db).await;
+    let cluster = |host: &str, password: &str| serde_json::json!({ "host": host, "port": 5432, "user": "wm_admin", "password": password });
+    sqlx::query(
+        "INSERT INTO global_settings (name, value) VALUES
+             ('external_instance_pg', $1),
+             ('external_instance_pg_state', '{\"databases\": {\"dt_a\": {\"success\": true}}}')",
+    )
+    .bind(cluster("pg-a.internal", "one"))
+    .execute(&db)
+    .await
+    .unwrap();
+    let current = BTreeMap::from([(
+        "external_instance_pg".to_string(),
+        cluster("pg-a.internal", "one"),
+    )]);
+    let sync = |value: serde_json::Value| {
+        let desired = BTreeMap::from([("external_instance_pg".to_string(), value)]);
+        let (db, current) = (db.clone(), current.clone());
+        async move {
+            windmill_common::instance_config::sync_global_settings_declarative(
+                &db, &current, &desired,
+            )
+            .await
+        }
+    };
+
+    let err = sync(cluster("pg-b.internal", "one"))
+        .await
+        .expect_err("another host must be refused while dt_a is registered");
+    assert!(err.to_string().contains("dt_a"), "got: {err}");
+    assert_eq!(
+        get_global_setting(&db, "external_instance_pg").await,
+        Some(cluster("pg-a.internal", "one"))
+    );
+
+    sync(cluster("PG-A.internal ", "two"))
+        .await
+        .expect("a new login on the same cluster must sync");
+}
diff --git a/backend/windmill-common/src/external_instance_pg.rs b/backend/windmill-common/src/external_instance_pg.rs
index 454e5c4b81..b52a847996 100644
--- a/backend/windmill-common/src/external_instance_pg.rs
+++ b/backend/windmill-common/src/external_instance_pg.rs
@@ -161,11 +161,39 @@ pub async fn check_external_instance_pg_write(
             ensure_external_instance_pg_removable(db).await
         }
         Some(value) => {
-            crate::external_instance_pg_oss::validate_external_instance_pg_setting(value)
+            crate::external_instance_pg_oss::validate_external_instance_pg_setting(value)?;
+            ensure_same_cluster_while_in_use(db, value).await
         }
     }
 }
 
+/// Pointing the setting at another cluster strands the databases Windmill created on this one as
+/// surely as unsetting it, and would hand their registry and role passwords to a cluster that has
+/// neither. The login, TLS and maintenance database may change; the host and port may not.
+async fn ensure_same_cluster_while_in_use(db: &DB, value: &serde_json::Value) -> Result<()> {
+    let state = read_external_instance_pg_state(db).await?;
+    if state.databases.is_empty() {
+        return Ok(());
+    }
+    let identity = |c: &ExternalInstancePg| (c.host.trim().to_lowercase(), c.port.unwrap_or(5432));
+    let new: ExternalInstancePg = serde_json::from_value(value.clone())
+        .map_err(|e| Error::BadRequest(format!("{EXTERNAL_INSTANCE_PG_SETTING}: {e}")))?;
+    let current = read_external_instance_pg_config(db).await?;
+    if current.is_some_and(|current| identity(&current) == identity(&new)) {
+        return Ok(());
+    }
+    Err(Error::BadRequest(format!(
+        "The external instance cluster still holds databases Windmill created ({}). Drop them \
+         before pointing {EXTERNAL_INSTANCE_PG_SETTING} at another host or port.",
+        state
+            .databases
+            .keys()
+            .cloned()
+            .collect::<Vec<_>>()
+            .join(", ")
+    )))
+}
+
 /// Converge the external cluster on the configured login: check what it can do, create or update
 /// Windmill's two roles with the stored passwords, and report anything that would get in the way.
 /// With `rotate_passwords`, generate new passwords first. Safe to run again; running it again is