diff --git a/.env.example b/.env.example
index 4b4edbf..0da689a 100644
--- a/.env.example
+++ b/.env.example
@@ -80,3 +80,8 @@ APP_API_PREFIX=/api/v1
 # Comma-separated list of allowed origins for the browser. Use specific origins
 # in production; * is accepted only for local development.
 CORS_ALLOWED_ORIGINS=http://localhost:5173,http://localhost:5273,http://localhost:4173
+
+# Optional shared-secret API key. When empty, the API is open (dev default).
+# When set, every request under APP_API_PREFIX except /health requires
+# X-API-Key: <value>  or  Authorization: Bearer <value>.
+API_KEY=
diff --git a/.env.prod.example b/.env.prod.example
index b7477f3..8503386 100644
--- a/.env.prod.example
+++ b/.env.prod.example
@@ -72,3 +72,6 @@ APP_API_PREFIX=/api/v1
 
 # Comma-separated list of allowed origins. NEVER use * in production.
 CORS_ALLOWED_ORIGINS=https://legacyhub.teamhub.example
+
+# Mandatory in production. Use a long random value (e.g. `openssl rand -hex 32`).
+API_KEY=__ROTATE_ME__
diff --git a/RUNBOOK.md b/RUNBOOK.md
index ea6712c..5d318d1 100644
--- a/RUNBOOK.md
+++ b/RUNBOOK.md
@@ -95,6 +95,36 @@ docker compose exec postgres psql -U legacyhub -d legacyhub -c \
 | Indexing stuck       | OpenSearch + Qdrant health                          | `scripts/init_opensearch.py`, `scripts/init_qdrant.py` |
 | Reranker disabled    | API logs → `reranker.disabled`                      | Ensure `RERANKER_ENABLED=true`; HF cache mounted |
 
+## API authentication
+
+Two mechanisms layered together:
+
+1. **Reverse proxy / SSO** (preferred). Front the API with nginx, Traefik, or
+   an OAuth gateway. The reverse proxy terminates TLS and authenticates the
+   caller; LegacyHUB never sees a raw user identity.
+2. **Shared-secret API key** (defence in depth). Set `API_KEY` to a long
+   random value (`openssl rand -hex 32`). Every request to `APP_API_PREFIX`
+   except `/health` must then carry either:
+
+   ```http
+   X-API-Key: <key>
+   ```
+   or:
+   ```http
+   Authorization: Bearer <key>
+   ```
+
+   `/health` is intentionally exempt so external probes do not need the
+   secret.
+
+   In production this is required (`docker-compose.prod.yml` fails the
+   stack if `API_KEY` is empty). In development the key is optional and
+   the default empty value disables the middleware entirely.
+
+   The frontend reads `VITE_API_KEY` and injects the header on every Axios
+   request. For SSO deployments leave `VITE_API_KEY` empty and let the
+   reverse proxy inject the header server-side.
+
 ## Verification gates (per change)
 
 1. `python -m pytest tests/ -q` — full unit suite (19+ tests).
diff --git a/app/api/security.py b/app/api/security.py
new file mode 100644
index 0000000..4637994
--- /dev/null
+++ b/app/api/security.py
@@ -0,0 +1,83 @@
+"""Optional API-key auth.
+
+Behaviour:
+
+- If ``API_KEY`` is empty (default) every request is allowed - matches the
+  original dev configuration.
+- If ``API_KEY`` is set, every request to a route under ``app_api_prefix``
+  must carry either ``X-API-Key: <value>`` or ``Authorization: Bearer <value>``.
+- ``/health`` is intentionally exempt so external probes (compose healthcheck,
+  reverse proxy, monitoring) keep working without leaking the key.
+- The root ``/`` page stays open so the OpenAPI banner and docs links remain
+  reachable.
+
+This is a defence-in-depth layer behind whatever reverse proxy / OAuth gateway
+runs in production - not a replacement.
+"""
+
+from __future__ import annotations
+
+import hmac
+from typing import Awaitable, Callable
+
+from fastapi import FastAPI, Request, Response
+from fastapi.responses import JSONResponse
+from starlette.types import ASGIApp
+
+from app.config import settings as _module_settings
+
+EXEMPT_PATHS: tuple[str, ...] = ("/", "/docs", "/redoc", "/openapi.json")
+EXEMPT_SUFFIXES: tuple[str, ...] = ("/health",)
+
+
+def _extract_token(request: Request) -> str | None:
+    header = request.headers.get("x-api-key")
+    if header:
+        return header.strip()
+    auth = request.headers.get("authorization") or ""
+    if auth.lower().startswith("bearer "):
+        return auth[7:].strip()
+    return None
+
+
+def install_api_key_auth(app: FastAPI) -> None:
+    """Attach the middleware. Always safe to call; becomes a no-op when no key
+    is configured.
+
+    Reads ``app.config.settings`` lazily so test fixtures can reload the config
+    module and have the new ``API_KEY`` value take effect on the next install.
+    """
+    from app.config import settings as fresh_settings  # re-resolve after reloads
+
+    settings = fresh_settings
+    expected = settings.api_key.strip() if settings.api_key else ""
+    if not expected:
+        return
+
+    @app.middleware("http")
+    async def _api_key_middleware(  # type: ignore[no-redef]
+        request: Request,
+        call_next: Callable[[Request], Awaitable[Response]],
+    ) -> Response:
+        path = request.url.path
+        if request.method == "OPTIONS":
+            return await call_next(request)
+        if path in EXEMPT_PATHS:
+            return await call_next(request)
+        if any(path.endswith(s) for s in EXEMPT_SUFFIXES):
+            return await call_next(request)
+        if not path.startswith(settings.app_api_prefix):
+            return await call_next(request)
+
+        token = _extract_token(request)
+        if not token or not hmac.compare_digest(token, expected):
+            return JSONResponse(
+                status_code=401,
+                content={"detail": "invalid or missing api key"},
+                headers={"WWW-Authenticate": "Bearer"},
+            )
+        return await call_next(request)
+
+
+__all__ = ["install_api_key_auth"]
+_ = ASGIApp  # re-export hint to keep mypy happy on older Starlette versions
diff --git a/app/config.py b/app/config.py
index ed8fe22..65e988d 100644
--- a/app/config.py
+++ b/app/config.py
@@ -31,6 +31,7 @@ class Settings(BaseSettings):
         "http://localhost:5173,http://localhost:5273,http://localhost:4173",
         alias="CORS_ALLOWED_ORIGINS",
     )
+    api_key: str = Field("", alias="API_KEY")
 
     @property
     def cors_origins(self) -> list[str]:
diff --git a/app/main.py b/app/main.py
index 5a896af..698c295 100644
--- a/app/main.py
+++ b/app/main.py
@@ -10,6 +10,7 @@ from fastapi.middleware.cors import CORSMiddleware
 
 from app import __version__
 from app.api import routes_health, routes_ingestion, routes_search
+from app.api.security import install_api_key_auth
 from app.config import settings
 from app.logging_config import configure_logging, get_logger
 
@@ -43,9 +44,10 @@ app.add_middleware(
     allow_origins=settings.cors_origins,
     allow_credentials=True,
     allow_methods=["GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS"],
-    allow_headers=["*"],
+    allow_headers=["*", "X-API-Key", "Authorization"],
     max_age=3600,
 )
+install_api_key_auth(app)
 
 app.include_router(routes_health.router, prefix=settings.app_api_prefix)
 app.include_router(routes_ingestion.router, prefix=settings.app_api_prefix)
diff --git a/docker-compose.prod.yml b/docker-compose.prod.yml
index 2a33c1d..c1ecbdc 100644
--- a/docker-compose.prod.yml
+++ b/docker-compose.prod.yml
@@ -92,6 +92,7 @@ services:
         APP_INPUT_DIR: /data/input
         APP_WORK_DIR: /data/work
         CORS_ALLOWED_ORIGINS: ${CORS_ALLOWED_ORIGINS:?CORS_ALLOWED_ORIGINS must be set (no * in production)}
+        API_KEY: ${API_KEY:?API_KEY must be set in production}
     restart: always
 
   worker:
diff --git a/docker-compose.yml b/docker-compose.yml
index 685fffd..be2f91a 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -33,6 +33,7 @@ x-common-env: &common-env
   APP_INPUT_DIR: /data/input
   APP_WORK_DIR: /data/work
   CORS_ALLOWED_ORIGINS: ${CORS_ALLOWED_ORIGINS:-http://localhost:5173,http://localhost:5273,http://localhost:4173}
+  API_KEY: ${API_KEY:-}
 
 services:
   postgres:
diff --git a/frontend/.env.example b/frontend/.env.example
index 110ece7..9739f60 100644
--- a/frontend/.env.example
+++ b/frontend/.env.example
@@ -2,3 +2,8 @@
 VITE_API_BASE_URL=/api/v1
 VITE_USE_MOCK=true
 VITE_APP_NAME=LegacyHUB
+
+# Optional. When the backend has API_KEY set, the SPA must echo it on every
+# request. For SSO/cookie deployments leave this empty and let the reverse
+# proxy inject the header server-side.
+VITE_API_KEY=
diff --git a/frontend/src/services/apiClient.ts b/frontend/src/services/apiClient.ts
index 7b6039e..bbab3c7 100644
--- a/frontend/src/services/apiClient.ts
+++ b/frontend/src/services/apiClient.ts
@@ -1,11 +1,15 @@
 import axios, { type AxiosInstance, type AxiosError } from "axios";
 
 const BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "/api/v1";
+const API_KEY = import.meta.env.VITE_API_KEY ?? "";
+
+const defaultHeaders: Record<string, string> = { "Content-Type": "application/json" };
+if (API_KEY) defaultHeaders["X-API-Key"] = API_KEY;
 
 export const apiClient: AxiosInstance = axios.create({
   baseURL: BASE_URL,
   timeout: 60_000,
-  headers: { "Content-Type": "application/json" },
+  headers: defaultHeaders,
 });
 
 apiClient.interceptors.response.use(
diff --git a/frontend/src/vite-env.d.ts b/frontend/src/vite-env.d.ts
index 85cdf64..d6953c4 100644
--- a/frontend/src/vite-env.d.ts
+++ b/frontend/src/vite-env.d.ts
@@ -4,6 +4,7 @@ interface ImportMetaEnv {
   readonly VITE_API_BASE_URL?: string;
   readonly VITE_USE_MOCK?: string;
   readonly VITE_APP_NAME?: string;
+  readonly VITE_API_KEY?: string;
 }
 
 interface ImportMeta {
diff --git a/tests/test_api_security.py b/tests/test_api_security.py
new file mode 100644
index 0000000..4d086c4
--- /dev/null
+++ b/tests/test_api_security.py
@@ -0,0 +1,167 @@
+"""Tests for the optional API-key auth middleware."""
+
+from __future__ import annotations
+
+import importlib
+
+import pytest
+
+from fastapi.testclient import TestClient
+
+
+KEY = "test-secret-key-DO-NOT-USE-IN-PROD"
+
+
+@pytest.fixture
+def secured_app(monkeypatch):
+    """Reload the FastAPI application with API_KEY set so the middleware
+    installs itself before the lifespan starts. Returns a TestClient bound to
+    that fresh app instance.
+    """
+    monkeypatch.setenv("API_KEY", KEY)
+
+    # Drop cached Settings and main so the new env vars are picked up.
+    import app.config as cfg
+    import app.main as main_module
+
+    cfg.get_settings.cache_clear()
+    importlib.reload(cfg)
+    importlib.reload(main_module)
+    return main_module.app
+
+
+def _patch_health(monkeypatch, module):
+    from app.api.schemas import ComponentHealth
+
+    def _ok(name):
+        return ComponentHealth(name=name, status="ok", detail={})
+
+    for name in (
+        "_check_postgres",
+        "_check_minio",
+        "_check_opensearch",
+        "_check_qdrant",
+        "_check_redis",
+    ):
+        monkeypatch.setattr(module, name, lambda n=name: _ok(n.removeprefix("_check_")))
+
+
+def test_health_remains_open_when_key_required(secured_app, monkeypatch):
+    from app.api import routes_health
+    from app.config import settings
+
+    _patch_health(monkeypatch, routes_health)
+    client = TestClient(secured_app)
+    res = client.get(f"{settings.app_api_prefix}/health")
+    assert res.status_code == 200
+
+
+def test_protected_route_rejects_missing_key(secured_app, monkeypatch):
+    from app.config import settings
+    from app.indexing import hybrid_search
+
+    monkeypatch.setattr(hybrid_search, "run_search", lambda req: pytest.fail("must not run"))
+
+    client = TestClient(secured_app)
+    res = client.post(
+        f"{settings.app_api_prefix}/search",
+        json={
+            "query": "anything",
+            "limit": 1,
+            "filters": {
+                "document_id": None,
+                "source_path": None,
+                "block_type": None,
+                "min_ocr_confidence": None,
+            },
+            "search_mode": "hybrid",
+        },
+    )
+    assert res.status_code == 401
+    assert res.json()["detail"].startswith("invalid")
+
+
+def test_protected_route_accepts_x_api_key_header(secured_app, monkeypatch):
+    from app.config import settings
+    from app.indexing import hybrid_search
+    from app.api.schemas import SearchResponse
+
+    monkeypatch.setattr(
+        hybrid_search,
+        "run_search",
+        lambda req: SearchResponse(
+            query=req.query, mode=req.search_mode, total_candidates=0, reranked=False, results=[]
+        ),
+    )
+
+    client = TestClient(secured_app)
+    res = client.post(
+        f"{settings.app_api_prefix}/search",
+        headers={"X-API-Key": KEY},
+        json={
+            "query": "x",
+            "limit": 1,
+            "filters": {
+                "document_id": None,
+                "source_path": None,
+                "block_type": None,
+                "min_ocr_confidence": None,
+            },
+            "search_mode": "hybrid",
+        },
+    )
+    assert res.status_code == 200
+
+
+def test_protected_route_accepts_bearer_token(secured_app, monkeypatch):
+    from app.config import settings
+    from app.indexing import hybrid_search
+    from app.api.schemas import SearchResponse
+
+    monkeypatch.setattr(
+        hybrid_search,
+        "run_search",
+        lambda req: SearchResponse(
+            query=req.query, mode=req.search_mode, total_candidates=0, reranked=False, results=[]
+        ),
+    )
+
+    client = TestClient(secured_app)
+    res = client.post(
+        f"{settings.app_api_prefix}/search",
+        headers={"Authorization": f"Bearer {KEY}"},
+        json={
+            "query": "x",
+            "limit": 1,
+            "filters": {
+                "document_id": None,
+                "source_path": None,
+                "block_type": None,
+                "min_ocr_confidence": None,
+            },
+            "search_mode": "hybrid",
+        },
+    )
+    assert res.status_code == 200
+
+
+def test_protected_route_rejects_wrong_key(secured_app):
+    from app.config import settings
+
+    client = TestClient(secured_app)
+    res = client.post(
+        f"{settings.app_api_prefix}/search",
+        headers={"X-API-Key": "wrong"},
+        json={
+            "query": "x",
+            "limit": 1,
+            "filters": {
+                "document_id": None,
+                "source_path": None,
+                "block_type": None,
+                "min_ocr_confidence": None,
+            },
+            "search_mode": "hybrid",
+        },
+    )
+    assert res.status_code == 401