diff --git a/backend/app/startup.py b/backend/app/startup.py
index e93ad08..cebf138 100644
--- a/backend/app/startup.py
+++ b/backend/app/startup.py
@@ -41,6 +41,7 @@ from app.tasks import (
     geo_re_resolve,
     health_check,
     history_sync,
+    rate_limiter_cleanup,
     session_cleanup,
 )
 from app.utils.async_utils import run_blocking
@@ -395,6 +396,7 @@ async def _stage_register_tasks(app: FastAPI, scheduler: AsyncIOScheduler) -> No
     - geo_re_resolve: Periodic re-resolution of stale records
     - history_sync: Periodic synchronization of ban history
     - session_cleanup: Periodic cleanup of expired sessions
+    - rate_limiter_cleanup: Periodic cleanup of expired rate-limiter entries
 
     Args:
         app: The FastAPI application instance.
@@ -407,5 +409,6 @@ async def _stage_register_tasks(app: FastAPI, scheduler: AsyncIOScheduler) -> No
     geo_re_resolve.register(app)
     history_sync.register(app)
     session_cleanup.register(app)
+    rate_limiter_cleanup.register(app)
 
-    log.info("startup_tasks_registered", count=7)
+    log.info("startup_tasks_registered", count=8)
diff --git a/backend/app/tasks/rate_limiter_cleanup.py b/backend/app/tasks/rate_limiter_cleanup.py
new file mode 100644
index 0000000..45719c5
--- /dev/null
+++ b/backend/app/tasks/rate_limiter_cleanup.py
@@ -0,0 +1,71 @@
+"""Rate limiter cleanup background task.
+
+Registers an APScheduler job that periodically removes expired rate-limit
+entries from the in-memory rate limiter. Without this cleanup, the
+rate-limiter state dictionary grows unbounded over long runtimes, eventually
+consuming excessive memory.
+
+The cleanup is conservative: it only removes IPs with no recent attempts
+(all timestamps outside the rate-limit window), so active or recently-active
+IPs are preserved.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import structlog
+
+if TYPE_CHECKING:
+    from fastapi import FastAPI
+
+log: structlog.stdlib.BoundLogger = structlog.get_logger()
+
+#: How often the cleanup job fires (seconds). Chosen to balance memory
+#: management against CPU overhead. A 30-minute interval handles typical
+#: brute-force attack patterns while staying lightweight.
+RATE_LIMITER_CLEANUP_INTERVAL: int = 30 * 60  # 30 minutes
+
+#: Stable APScheduler job ID — ensures re-registration replaces, not duplicates.
+JOB_ID: str = "rate_limiter_cleanup"
+
+
+def _run_cleanup(app: FastAPI) -> None:
+    """Trigger cleanup of expired rate-limiter entries.
+
+    Args:
+        app: The FastAPI application instance (holds the rate limiter).
+    """
+    rate_limiter = getattr(app.state, "login_rate_limiter", None)
+    if rate_limiter is None:
+        log.warning(
+            "rate_limiter_cleanup_skipped",
+            reason="rate_limiter not found on app.state",
+        )
+        return
+
+    rate_limiter.cleanup_expired()
+
+
+def register(app: FastAPI) -> None:
+    """Add (or replace) the rate-limiter cleanup job in the application scheduler.
+
+    Must be called after the scheduler has been started (i.e., inside the
+    lifespan handler, after ``scheduler.start()``).
+
+    Args:
+        app: The :class:`fastapi.FastAPI` application instance whose
+            ``app.state.scheduler`` will receive the job.
+    """
+    app.state.scheduler.add_job(
+        _run_cleanup,
+        trigger="interval",
+        seconds=RATE_LIMITER_CLEANUP_INTERVAL,
+        kwargs={"app": app},
+        id=JOB_ID,
+        replace_existing=True,
+    )
+    log.info(
+        "rate_limiter_cleanup_scheduled",
+        interval_seconds=RATE_LIMITER_CLEANUP_INTERVAL,
+    )
diff --git a/backend/app/utils/rate_limiter.py b/backend/app/utils/rate_limiter.py
index cc004b5..1ff00df 100644
--- a/backend/app/utils/rate_limiter.py
+++ b/backend/app/utils/rate_limiter.py
@@ -11,6 +11,23 @@ attacks to a single worker.
 The penalty strategy for failed login attempts is also managed here:
 record_failure() records a failure timestamp and returns the penalty delay
 to apply, enabling progressive back-off without exhausting request capacity.
+
+Operational Notes
+-----------------
+
+**Cleanup Lifecycle**: The rate limiter state (_attempts, _failures, _lock_counts)
+grows as IPs interact with the system. To prevent unbounded memory growth during
+long runtimes, a scheduled background task (rate_limiter_cleanup) calls the
+cleanup_expired() method every 30 minutes. This is safe because:
+
+- cleanup_expired() only removes IPs with no recent attempts (all timestamps
+  outside the rate-limit window), so active IPs are never disrupted.
+- The cleanup is non-blocking and logged for observability.
+- Individual requests already prune old timestamps from each IP's deque during
+  is_allowed() and record_failure(), so cleanup primarily handles dormant IPs.
+
+For monitoring, check logs for "rate_limiter_cleanup" events to observe how
+many IPs are being retired from memory each cleanup cycle.
 """
 
 from __future__ import annotations