diff --git a/.env.example b/.env.example
index 6a44801..ea7653a 100644
--- a/.env.example
+++ b/.env.example
@@ -12,6 +12,7 @@ NEO4J_DATABASE=neo4j
 GITHUB_APP_ID=your_app_id_here
 GITHUB_PRIVATE_KEY_PATH=/path/to/your-app-private-key.pem
 GITHUB_WEBHOOK_SECRET=your_webhook_secret_here
+GITHUB_MARKETPLACE_WEBHOOK_SECRET=your_marketplace_webhook_secret_here
 
 # ============================================================================
 # Firebase (service account — JSON string or file path)
diff --git a/src/api/middleware/webhook_auth.py b/src/api/middleware/webhook_auth.py
new file mode 100644
index 0000000..387db5a
--- /dev/null
+++ b/src/api/middleware/webhook_auth.py
@@ -0,0 +1,49 @@
+import hashlib
+import hmac
+import logging
+import os
+
+from fastapi import HTTPException, Request
+
+logger = logging.getLogger(__name__)
+
+
+def _verify_signature(raw_body: bytes, secret: str, signature_header: str) -> None:
+    if not signature_header or not signature_header.startswith("sha256="):
+        raise HTTPException(status_code=403, detail="Missing or malformed X-Hub-Signature-256 header")
+    local_hash = hmac.new(
+        key=secret.encode("utf-8"),
+        msg=raw_body,
+        digestmod=hashlib.sha256,
+    ).hexdigest()
+    if not hmac.compare_digest(local_hash, signature_header.removeprefix("sha256=")):
+        raise HTTPException(status_code=403, detail="Invalid webhook signature")
+
+
+async def verify_github_webhook_signature(request: Request) -> None:
+    secret = os.environ.get("GITHUB_WEBHOOK_SECRET")
+    if not secret:
+        logger.error("GITHUB_WEBHOOK_SECRET is not set")
+        raise HTTPException(status_code=500, detail="Webhook secret not configured")
+    _verify_signature(
+        await request.body(),
+        secret,
+        request.headers.get("X-Hub-Signature-256", ""),
+    )
+
+
+async def verify_marketplace_webhook_signature(request: Request) -> None:
+    secret = os.environ.get("GITHUB_MARKETPLACE_WEBHOOK_SECRET")
+    if not secret:
+        logger.warning(
+            "GITHUB_MARKETPLACE_WEBHOOK_SECRET is not set — falling back to GITHUB_WEBHOOK_SECRET"
+        )
+        secret = os.environ.get("GITHUB_WEBHOOK_SECRET")
+    if not secret:
+        logger.error("Neither GITHUB_MARKETPLACE_WEBHOOK_SECRET nor GITHUB_WEBHOOK_SECRET is set")
+        raise HTTPException(status_code=500, detail="Webhook secret not configured")
+    _verify_signature(
+        await request.body(),
+        secret,
+        request.headers.get("X-Hub-Signature-256", ""),
+    )
\ No newline at end of file
diff --git a/src/api/routers/webhook.py b/src/api/routers/webhook.py
index 5265bc8..97c641f 100644
--- a/src/api/routers/webhook.py
+++ b/src/api/routers/webhook.py
@@ -2,8 +2,12 @@
 import logging
 import uuid
 
-from fastapi import APIRouter, BackgroundTasks, Request
+from fastapi import APIRouter, BackgroundTasks, Depends, Request
 
+from api.middleware.webhook_auth import (
+    verify_github_webhook_signature,
+    verify_marketplace_webhook_signature,
+)
 from api.services.cloud_tasks_service import CloudTasksService
 from api.services.code_review_commands import extract_review_command, is_bot_mentioned
 from common.firebase_service import firebase_service
@@ -22,7 +26,7 @@
 cloud_tasks = CloudTasksService()
 
 
-@router.post("/onComment")
+@router.post("/onComment", dependencies=[Depends(verify_github_webhook_signature)])
 async def on_comment(request: Request, background_tasks: BackgroundTasks):
     """
     Single GitHub webhook endpoint. Routes all events by X-GitHub-Event header:
@@ -289,7 +293,7 @@ async def _handle_comment_review(payload: dict, background_tasks: BackgroundTask
 }
 
 
-@router.post("/marketplace")
+@router.post("/marketplace", dependencies=[Depends(verify_marketplace_webhook_signature)])
 async def on_marketplace(request: Request):
     """
     GitHub Marketplace webhook.