diff --git a/bench_cli/managers/mariadb_manager.py b/bench_cli/managers/mariadb_manager.py
index 8fe1e02..dc962f4 100644
--- a/bench_cli/managers/mariadb_manager.py
+++ b/bench_cli/managers/mariadb_manager.py
@@ -1,4 +1,5 @@
 import shutil
+import socket
 from pathlib import Path
 
 from bench_cli.config.mariadb_config import MariaDBConfig
@@ -23,7 +24,16 @@ def install(self) -> None:
         package = self._brew_package() if is_macos() else self._apt_package()
         package_manager.install(package)
 
+    def is_running(self) -> bool:
+        try:
+            with socket.create_connection((self.config.host, self.config.port), timeout=1):
+                return True
+        except OSError:
+            return False
+
     def start(self) -> None:
+        if self.is_running():
+            return
         if is_macos():
             run_command(["brew", "services", "start", self._brew_package()])
         else:
diff --git a/tests/test_mariadb_manager.py b/tests/test_mariadb_manager.py
new file mode 100644
index 0000000..625256e
--- /dev/null
+++ b/tests/test_mariadb_manager.py
@@ -0,0 +1,80 @@
+"""Tests for MariaDBManager.is_running() and the guarded start()."""
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+from bench_cli.config.mariadb_config import MariaDBConfig
+from bench_cli.managers.mariadb_manager import MariaDBManager
+
+
+def make_manager(host: str = "localhost", port: int = 3306) -> MariaDBManager:
+    return MariaDBManager(MariaDBConfig(host=host, port=port))
+
+
+# ── is_running() ──────────────────────────────────────────────────────────────
+
+
+def test_is_running_returns_true_when_port_is_open() -> None:
+    manager = make_manager()
+    mock_conn = MagicMock()
+    with patch("socket.create_connection", return_value=mock_conn) as mock_create:
+        assert manager.is_running() is True
+        mock_create.assert_called_once_with(("localhost", 3306), timeout=1)
+
+
+def test_is_running_returns_false_when_connection_refused() -> None:
+    manager = make_manager()
+    with patch("socket.create_connection", side_effect=OSError("Connection refused")):
+        assert manager.is_running() is False
+
+
+def test_is_running_returns_false_on_timeout() -> None:
+    manager = make_manager()
+    with patch("socket.create_connection", side_effect=OSError("timed out")):
+        assert manager.is_running() is False
+
+
+def test_is_running_uses_configured_host_and_port() -> None:
+    manager = make_manager(host="127.0.0.1", port=3307)
+    mock_conn = MagicMock()
+    with patch("socket.create_connection", return_value=mock_conn) as mock_create:
+        manager.is_running()
+        mock_create.assert_called_once_with(("127.0.0.1", 3307), timeout=1)
+
+
+# ── start() ───────────────────────────────────────────────────────────────────
+
+
+def test_start_skips_service_when_already_running() -> None:
+    manager = make_manager()
+    with patch.object(manager, "is_running", return_value=True):
+        with patch("bench_cli.managers.mariadb_manager.run_command") as mock_run:
+            manager.start()
+            mock_run.assert_not_called()
+
+
+def test_start_calls_brew_on_macos_when_not_running() -> None:
+    manager = make_manager()
+    with patch.object(manager, "is_running", return_value=False):
+        with patch("bench_cli.managers.mariadb_manager.is_macos", return_value=True):
+            with patch("bench_cli.managers.mariadb_manager.run_command") as mock_run:
+                manager.start()
+                mock_run.assert_called_once_with(["brew", "services", "start", "mariadb"])
+
+
+def test_start_calls_brew_with_versioned_package() -> None:
+    manager = MariaDBManager(MariaDBConfig(host="localhost", port=3306, version="10.6"))
+    with patch.object(manager, "is_running", return_value=False):
+        with patch("bench_cli.managers.mariadb_manager.is_macos", return_value=True):
+            with patch("bench_cli.managers.mariadb_manager.run_command") as mock_run:
+                manager.start()
+                mock_run.assert_called_once_with(["brew", "services", "start", "mariadb@10.6"])
+
+
+def test_start_calls_systemctl_on_linux_when_not_running() -> None:
+    manager = make_manager()
+    with patch.object(manager, "is_running", return_value=False):
+        with patch("bench_cli.managers.mariadb_manager.is_macos", return_value=False):
+            with patch("bench_cli.managers.mariadb_manager.run_command") as mock_run:
+                manager.start()
+                mock_run.assert_called_once_with(["sudo", "systemctl", "start", "mariadb"])