diff --git a/repeater/web/api_endpoints.py b/repeater/web/api_endpoints.py
index a4a463a..fa6b480 100644
--- a/repeater/web/api_endpoints.py
+++ b/repeater/web/api_endpoints.py
@@ -2822,6 +2822,13 @@ class APIEndpoints:
                         "host": str(b["host"]).strip(),
                         "port": port,
                         "format": str(b["format"]).strip(),
+                        # Custom base topic for legacy `mqtt` format. Persist
+                        # None when blank so the handler can apply defaults.
+                        "base_topic": (
+                            (str(b["base_topic"]).strip() or None)
+                            if "base_topic" in b and b.get("base_topic") is not None
+                            else None
+                        ),
                         "disallowed_packet_types": list(b.get("disallowed_packet_types", [])),
                         "retain_status": bool(b.get("retain_status", False)),
                         # Opt-in per broker; brokers that do not expect the
diff --git a/tests/test_api_endpoints_core_coverage.py b/tests/test_api_endpoints_core_coverage.py
index e076775..9aed860 100644
--- a/tests/test_api_endpoints_core_coverage.py
+++ b/tests/test_api_endpoints_core_coverage.py
@@ -2593,13 +2593,22 @@ def test_update_mqtt_config_validation_and_success(cherrypy_ctx):
         "status_interval": 20,
         "brokers": [
             {"preset": "waev"},
-            {"name": "a", "host": "h", "port": 443, "format": "json", "enabled": True},
+            {
+                "name": "a",
+                "host": "h",
+                "port": 443,
+                "format": "mqtt",
+                "enabled": True,
+                "base_topic": "meshcore/custom/base",
+            },
         ],
     }
     api.config_manager.update_and_save.return_value = {"success": True, "saved": True}
     out = api.update_mqtt_config()
     assert out["success"] is True
     assert out["data"]["restart_required"] is True
+    updates = api.config_manager.update_and_save.call_args.kwargs["updates"]["mqtt_brokers"]
+    assert updates["brokers"][1]["base_topic"] == "meshcore/custom/base"
 
     api.config_manager.update_and_save.return_value = {"success": False, "error": "save failed"}
     out2 = api.update_mqtt_config()
diff --git a/tests/test_mqtt_publish_integration.py b/tests/test_mqtt_publish_integration.py
index 0e42ffb..43826fa 100644
--- a/tests/test_mqtt_publish_integration.py
+++ b/tests/test_mqtt_publish_integration.py
@@ -236,3 +236,39 @@ def test_mqtt_published_packet_legacy_mqtt_format_uses_singular_packet_topic():
     payload_dict = json.loads(captured[0]["payload"])
     # Duration still flows through correctly even on the legacy topic
     assert int(payload_dict["duration"]) > 0
+
+
+def test_mqtt_published_packet_legacy_mqtt_format_uses_supplied_base_topic():
+    """When a broker supplies ``base_topic``, downstream publishing must use
+    that value instead of the legacy default prefix.
+    """
+    config = _make_config(format_value="mqtt", iata_code="LAX")
+    config["mqtt_brokers"]["brokers"][0]["base_topic"] = "site/edge/gw1"
+    public_key_hex = "12" * 32
+    identity = _FakeIdentity(public_key_hex)
+
+    pusher = MeshCoreToMqttPusher(local_identity=identity, config=config)
+    captured = _attach_capturing_client(pusher.connections[0])
+
+    raw_bytes = bytes(range(16))
+    airtime_mgr = AirtimeManager(config)
+    packet_record = {
+        "timestamp": 1700000000.0,
+        "type": 1,
+        "route": 1,
+        "rssi": -70,
+        "snr": 6.0,
+        "score": 0.2,
+        "payload_length": 10,
+        "packet_hash": "C0FFEE00" + "00" * 4,
+        "raw_packet": raw_bytes.hex(),
+        "airtime_ms": airtime_mgr.calculate_airtime(len(raw_bytes)),
+    }
+    record = PacketRecord.from_packet_record(
+        packet_record, origin="test-node", origin_id=public_key_hex.upper()
+    )
+    pusher.publish_packet(record.to_dict())
+
+    assert len(captured) == 1
+    # Legacy `mqtt` format still uses singular subtopic, but custom base prefix.
+    assert captured[0]["topic"] == "site/edge/gw1/packet"