diff --git a/fablednetmon/core/cleanup.py b/fablednetmon/core/cleanup.py
new file mode 100644
index 0000000..07340a7
--- /dev/null
+++ b/fablednetmon/core/cleanup.py
@@ -0,0 +1,33 @@
+from __future__ import annotations
+import logging
+from datetime import datetime, timedelta, timezone
+from typing import TYPE_CHECKING
+
+from sqlalchemy import delete
+
+from fablednetmon.models.monitors import DnsResult, PingResult
+from fablednetmon.models.metrics import PluginMetric
+
+if TYPE_CHECKING:
+    from quart import Quart
+
+logger = logging.getLogger(__name__)
+
+
+async def run_cleanup(app: "Quart") -> None:
+    """Delete rows older than DATA_RETENTION_DAYS from time-series tables."""
+    retention_days: int = app.config.get("DATA_RETENTION_DAYS", 90)
+    cutoff = datetime.now(timezone.utc) - timedelta(days=retention_days)
+
+    async with app.db_sessionmaker() as session:
+        async with session.begin():
+            for model, ts_col in [
+                (PingResult, PingResult.probed_at),
+                (DnsResult, DnsResult.resolved_at),
+                (PluginMetric, PluginMetric.recorded_at),
+            ]:
+                result = await session.execute(
+                    delete(model).where(ts_col < cutoff)
+                )
+                if result.rowcount:
+                    logger.info(f"Pruned {result.rowcount} rows from {model.__tablename__}")