diff --git a/.env.example b/.env.example
index b0448ff2..825bf6cf 100644
--- a/.env.example
+++ b/.env.example
@@ -27,6 +27,10 @@ SECUSCAN_ALLOW_LOOPBACK_SCANS=true
 # The server refuses to start the vault if this is unset.
 SECUSCAN_VAULT_KEY=replace-with-output-of-secrets.token_hex-32
 
+# API key for authenticating all API requests (auto-generated on first startup if not set)
+# Generate with: python -c "import secrets; print(secrets.token_hex(32))"
+SECUSCAN_API_KEY=
+
 # Plugin Security
 # SECUSCAN_PLUGIN_SIGNATURE_KEY=replace-with-your-signing-key
 # SECUSCAN_ENFORCE_PLUGIN_SIGNATURES=false
diff --git a/backend/secuscan/auth.py b/backend/secuscan/auth.py
new file mode 100644
index 00000000..78acc2d5
--- /dev/null
+++ b/backend/secuscan/auth.py
@@ -0,0 +1,60 @@
+"""
+API key authentication dependency for SecuScan.
+
+On first startup, if SECUSCAN_API_KEY is not set, a random key is generated,
+written to data/api_key.txt, and logged once so the user can copy it.
+Every non-health-check request must supply the key as:
+  - Authorization: Bearer <key>   OR
+  - X-Api-Key: <key>
+"""
+from __future__ import annotations
+
+import logging
+import secrets
+from functools import lru_cache
+from pathlib import Path
+
+from fastapi import Depends, HTTPException, Request, status
+from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
+
+logger = logging.getLogger(__name__)
+
+_bearer = HTTPBearer(auto_error=False)
+
+_API_KEY_FILE = Path(__file__).resolve().parent.parent / "data" / "api_key.txt"
+
+
+@lru_cache(maxsize=1)
+def _get_active_key() -> str:
+    from .config import settings
+    if settings.api_key:
+        return settings.api_key
+    if _API_KEY_FILE.exists():
+        key = _API_KEY_FILE.read_text().strip()
+        if key:
+            return key
+    key = secrets.token_hex(32)
+    _API_KEY_FILE.parent.mkdir(parents=True, exist_ok=True)
+    _API_KEY_FILE.write_text(key)
+    logger.warning(
+        "No SECUSCAN_API_KEY set. Generated key saved to %s -- API KEY: %s",
+        _API_KEY_FILE, key,
+    )
+    return key
+
+
+async def require_api_key(
+    request: Request,
+    credentials: HTTPAuthorizationCredentials | None = Depends(_bearer),
+) -> None:
+    active_key = _get_active_key()
+    if credentials and secrets.compare_digest(credentials.credentials, active_key):
+        return
+    x_api_key = request.headers.get("X-Api-Key", "")
+    if x_api_key and secrets.compare_digest(x_api_key, active_key):
+        return
+    raise HTTPException(
+        status_code=status.HTTP_401_UNAUTHORIZED,
+        detail="Invalid or missing API key.",
+        headers={"WWW-Authenticate": "Bearer"},
+    )
diff --git a/backend/secuscan/config.py b/backend/secuscan/config.py
index 3b67d255..30c954cf 100644
--- a/backend/secuscan/config.py
+++ b/backend/secuscan/config.py
@@ -58,6 +58,7 @@ class Settings(BaseSettings):
     plugin_signature_key: Optional[str] = None
     enforce_plugin_signatures: bool = False
     vault_key: Optional[str] = None
+    api_key: Optional[str] = None
 
     # Rate Limiting
     max_concurrent_tasks: int = 3
diff --git a/backend/secuscan/main.py b/backend/secuscan/main.py
index ebf57f60..a9a5f533 100644
--- a/backend/secuscan/main.py
+++ b/backend/secuscan/main.py
@@ -9,7 +9,7 @@
 from contextlib import asynccontextmanager
 from .request_middleware import RequestIDMiddleware
 
-from fastapi import FastAPI
+from fastapi import Depends, FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.staticfiles import StaticFiles
 
@@ -18,6 +18,7 @@
 from .database import init_db, db as global_db
 from .plugins import init_plugins
 from .routes import router
+from .auth import require_api_key
 from .workflows import scheduler
 
 
@@ -124,7 +125,7 @@ async def redirect_api_openapi():
 app.add_middleware(RequestIDMiddleware)
 
 # Include API routes
-app.include_router(router)
+app.include_router(router, dependencies=[Depends(require_api_key)])
 
 # Health check endpoint
 @app.get("/api/v1/health")
diff --git a/testing/backend/conftest.py b/testing/backend/conftest.py
index 22e576b5..941cbb19 100644
--- a/testing/backend/conftest.py
+++ b/testing/backend/conftest.py
@@ -34,6 +34,9 @@ def setup_test_environment(monkeypatch):
     monkeypatch.setattr(settings, "plugins_dir", str(repo_root / "plugins"))
     monkeypatch.setattr(settings, "database_path", f"{temp_path}/test_secuscan.db")
     monkeypatch.setattr(settings, "vault_key", "test-vault-key-for-unit-tests-only")
+    monkeypatch.setattr(settings, "api_key", "test-api-key-for-unit-tests-only")
+    from backend.secuscan.auth import _get_active_key
+    _get_active_key.cache_clear()
 
     settings.ensure_directories()