diff --git a/supabase_auth/_sync/client.py b/supabase_auth/_sync/client.py
index 7f075765..cf579984 100644
--- a/supabase_auth/_sync/client.py
+++ b/supabase_auth/_sync/client.py
@@ -10,7 +10,7 @@
 
 from ..constants import COOKIE_OPTIONS, DEFAULT_HEADERS, GOTRUE_URL, STORAGE_KEY
 from ..exceptions import APIError
-from ..helpers import is_http_url, model_dump, model_validate
+from ..helpers import is_http_url, is_valid_jwt, model_dump, model_validate
 from ..types import (
     AuthChangeEvent,
     CookieOptions,
@@ -364,6 +364,10 @@ def set_session(self, *, refresh_token: str) -> Session:
         APIError
             If an error occurs.
         """
+
+        if not is_valid_jwt(refresh_token):
+            ValueError("refresh_token must be a valid JWT authorization token")
+
         response = self.api.refresh_access_token(refresh_token=refresh_token)
         self._save_session(session=response)
         self._notify_all_subscribers(event=AuthChangeEvent.SIGNED_IN)
@@ -388,6 +392,10 @@ def set_auth(self, *, access_token: str) -> Session:
         APIError
             If an error occurs.
         """
+
+        if not is_valid_jwt(access_token):
+            ValueError("access_token must be a valid JWT authorization token")
+
         session = Session(
             access_token=access_token,
             token_type="bearer",
diff --git a/supabase_auth/helpers.py b/supabase_auth/helpers.py
index 9808efd1..ae9dc7c5 100644
--- a/supabase_auth/helpers.py
+++ b/supabase_auth/helpers.py
@@ -35,6 +35,7 @@
 )
 
 TBaseModel = TypeVar("TBaseModel", bound=BaseModel)
+BASE64URL_REGEX = r"^([a-z0-9_-]{4})*($|[a-z0-9_-]{3}$|[a-z0-9_-]{2}$)$"
 
 
 def model_validate(model: Type[TBaseModel], contents) -> TBaseModel:
@@ -243,3 +244,26 @@ def parse_response_api_version(response: Response):
 
 def is_http_url(url: str) -> bool:
     return urlparse(url).scheme in {"https", "http"}
+
+
+def is_valid_jwt(value: str) -> bool:
+    """Checks if value looks like a JWT, does not do any extra parsing."""
+    if not isinstance(value, str):
+        return False
+
+    # Remove trailing whitespaces if any.
+    value = value.strip()
+
+    # Remove "Bearer " prefix if any.
+    if value.startswith("Bearer "):
+        value = value[7:]
+
+    # Valid JWT must have 2 dots (Header.Paylod.Signature)
+    if value.count(".") != 2:
+        return False
+
+    for part in value.split("."):
+        if not re.search(BASE64URL_REGEX, part, re.IGNORECASE):
+            return False
+
+    return True