diff --git a/surfsense_backend/app/routes/google_calendar_add_connector_route.py b/surfsense_backend/app/routes/google_calendar_add_connector_route.py
index 9a2308bec..725f8decc 100644
--- a/surfsense_backend/app/routes/google_calendar_add_connector_route.py
+++ b/surfsense_backend/app/routes/google_calendar_add_connector_route.py
@@ -28,7 +28,7 @@ from app.utils.connector_naming import (
     check_duplicate_connector,
     generate_unique_connector_name,
 )
-from app.utils.oauth_security import OAuthStateManager, TokenEncryption
+from app.utils.oauth_security import OAuthStateManager, TokenEncryption, generate_code_verifier
 
 logger = logging.getLogger(__name__)
 
@@ -96,9 +96,14 @@ async def connect_calendar(space_id: int, user: User = Depends(current_active_us
 
         flow = get_google_flow()
 
-        # Generate secure state parameter with HMAC signature
+        code_verifier = generate_code_verifier()
+        flow.code_verifier = code_verifier
+
+        # Generate secure state parameter with HMAC signature (includes PKCE code_verifier)
         state_manager = get_state_manager()
-        state_encoded = state_manager.generate_secure_state(space_id, user.id)
+        state_encoded = state_manager.generate_secure_state(
+            space_id, user.id, code_verifier=code_verifier
+        )
 
         auth_url, _ = flow.authorization_url(
             access_type="offline",
@@ -146,8 +151,11 @@ async def reauth_calendar(
 
         flow = get_google_flow()
 
+        code_verifier = generate_code_verifier()
+        flow.code_verifier = code_verifier
+
         state_manager = get_state_manager()
-        extra: dict = {"connector_id": connector_id}
+        extra: dict = {"connector_id": connector_id, "code_verifier": code_verifier}
         if return_url and return_url.startswith("/"):
             extra["return_url"] = return_url
         state_encoded = state_manager.generate_secure_state(space_id, user.id, **extra)
@@ -225,6 +233,7 @@ async def calendar_callback(
 
         user_id = UUID(data["user_id"])
         space_id = data["space_id"]
+        code_verifier = data.get("code_verifier")
 
         # Validate redirect URI (security: ensure it matches configured value)
         if not config.GOOGLE_CALENDAR_REDIRECT_URI:
@@ -233,6 +242,7 @@ async def calendar_callback(
             )
 
         flow = get_google_flow()
+        flow.code_verifier = code_verifier
         flow.fetch_token(code=code)
 
         creds = flow.credentials
diff --git a/surfsense_backend/app/routes/google_drive_add_connector_route.py b/surfsense_backend/app/routes/google_drive_add_connector_route.py
index 1c9391610..921f84af9 100644
--- a/surfsense_backend/app/routes/google_drive_add_connector_route.py
+++ b/surfsense_backend/app/routes/google_drive_add_connector_route.py
@@ -41,7 +41,7 @@ from app.utils.connector_naming import (
     check_duplicate_connector,
     generate_unique_connector_name,
 )
-from app.utils.oauth_security import OAuthStateManager, TokenEncryption
+from app.utils.oauth_security import OAuthStateManager, TokenEncryption, generate_code_verifier
 
 # Relax token scope validation for Google OAuth
 os.environ["OAUTHLIB_RELAX_TOKEN_SCOPE"] = "1"
@@ -127,14 +127,19 @@ async def connect_drive(space_id: int, user: User = Depends(current_active_user)
 
         flow = get_google_flow()
 
-        # Generate secure state parameter with HMAC signature
+        code_verifier = generate_code_verifier()
+        flow.code_verifier = code_verifier
+
+        # Generate secure state parameter with HMAC signature (includes PKCE code_verifier)
         state_manager = get_state_manager()
-        state_encoded = state_manager.generate_secure_state(space_id, user.id)
+        state_encoded = state_manager.generate_secure_state(
+            space_id, user.id, code_verifier=code_verifier
+        )
 
         # Generate authorization URL
         auth_url, _ = flow.authorization_url(
-            access_type="offline",  # Get refresh token
-            prompt="consent",  # Force consent screen to get refresh token
+            access_type="offline",
+            prompt="consent",
             include_granted_scopes="true",
             state=state_encoded,
         )
@@ -193,8 +198,11 @@ async def reauth_drive(
 
         flow = get_google_flow()
 
+        code_verifier = generate_code_verifier()
+        flow.code_verifier = code_verifier
+
         state_manager = get_state_manager()
-        extra: dict = {"connector_id": connector_id}
+        extra: dict = {"connector_id": connector_id, "code_verifier": code_verifier}
         if return_url and return_url.startswith("/"):
             extra["return_url"] = return_url
         state_encoded = state_manager.generate_secure_state(space_id, user.id, **extra)
@@ -285,6 +293,7 @@ async def drive_callback(
         space_id = data["space_id"]
         reauth_connector_id = data.get("connector_id")
         reauth_return_url = data.get("return_url")
+        code_verifier = data.get("code_verifier")
 
         logger.info(
             f"Processing Google Drive callback for user {user_id}, space {space_id}"
@@ -296,8 +305,9 @@ async def drive_callback(
                 status_code=500, detail="GOOGLE_DRIVE_REDIRECT_URI not configured"
             )
 
-        # Exchange authorization code for tokens
+        # Exchange authorization code for tokens (restore PKCE code_verifier from state)
         flow = get_google_flow()
+        flow.code_verifier = code_verifier
         flow.fetch_token(code=code)
 
         creds = flow.credentials
diff --git a/surfsense_backend/app/routes/google_gmail_add_connector_route.py b/surfsense_backend/app/routes/google_gmail_add_connector_route.py
index 750a64819..9fe0c0de6 100644
--- a/surfsense_backend/app/routes/google_gmail_add_connector_route.py
+++ b/surfsense_backend/app/routes/google_gmail_add_connector_route.py
@@ -28,7 +28,7 @@ from app.utils.connector_naming import (
     check_duplicate_connector,
     generate_unique_connector_name,
 )
-from app.utils.oauth_security import OAuthStateManager, TokenEncryption
+from app.utils.oauth_security import OAuthStateManager, TokenEncryption, generate_code_verifier
 
 logger = logging.getLogger(__name__)
 
@@ -109,9 +109,14 @@ async def connect_gmail(space_id: int, user: User = Depends(current_active_user)
 
         flow = get_google_flow()
 
-        # Generate secure state parameter with HMAC signature
+        code_verifier = generate_code_verifier()
+        flow.code_verifier = code_verifier
+
+        # Generate secure state parameter with HMAC signature (includes PKCE code_verifier)
         state_manager = get_state_manager()
-        state_encoded = state_manager.generate_secure_state(space_id, user.id)
+        state_encoded = state_manager.generate_secure_state(
+            space_id, user.id, code_verifier=code_verifier
+        )
 
         auth_url, _ = flow.authorization_url(
             access_type="offline",
@@ -164,8 +169,11 @@ async def reauth_gmail(
 
         flow = get_google_flow()
 
+        code_verifier = generate_code_verifier()
+        flow.code_verifier = code_verifier
+
         state_manager = get_state_manager()
-        extra: dict = {"connector_id": connector_id}
+        extra: dict = {"connector_id": connector_id, "code_verifier": code_verifier}
         if return_url and return_url.startswith("/"):
             extra["return_url"] = return_url
         state_encoded = state_manager.generate_secure_state(space_id, user.id, **extra)
@@ -256,6 +264,7 @@ async def gmail_callback(
 
         user_id = UUID(data["user_id"])
         space_id = data["space_id"]
+        code_verifier = data.get("code_verifier")
 
         # Validate redirect URI (security: ensure it matches configured value)
         if not config.GOOGLE_GMAIL_REDIRECT_URI:
@@ -264,6 +273,7 @@ async def gmail_callback(
             )
 
         flow = get_google_flow()
+        flow.code_verifier = code_verifier
         flow.fetch_token(code=code)
 
         creds = flow.credentials
diff --git a/surfsense_backend/app/utils/oauth_security.py b/surfsense_backend/app/utils/oauth_security.py
index 5135cdef4..0ad9d3bd9 100644
--- a/surfsense_backend/app/utils/oauth_security.py
+++ b/surfsense_backend/app/utils/oauth_security.py
@@ -11,6 +11,8 @@ import hmac
 import json
 import logging
 import time
+from random import SystemRandom
+from string import ascii_letters, digits
 from uuid import UUID
 
 from cryptography.fernet import Fernet
@@ -18,6 +20,14 @@ from fastapi import HTTPException
 
 logger = logging.getLogger(__name__)
 
+_PKCE_CHARS = ascii_letters + digits + "-._~"
+_PKCE_RNG = SystemRandom()
+
+
+def generate_code_verifier(length: int = 128) -> str:
+    """Generate a PKCE code_verifier (RFC 7636, 43-128 unreserved chars)."""
+    return "".join(_PKCE_RNG.choice(_PKCE_CHARS) for _ in range(length))
+
 
 class OAuthStateManager:
     """Manages secure OAuth state parameters with HMAC signatures."""