diff --git a/changelog/325.fixed.md b/changelog/325.fixed.md
new file mode 100644
index 00000000..8acb0fe8
--- /dev/null
+++ b/changelog/325.fixed.md
@@ -0,0 +1 @@
+Fixed diff data url query params missing "?" character
diff --git a/infrahub_sdk/branch.py b/infrahub_sdk/branch.py
index 4c89bd41..bd552199 100644
--- a/infrahub_sdk/branch.py
+++ b/infrahub_sdk/branch.py
@@ -79,7 +79,7 @@ def generate_diff_data_url(
         if time_to:
             url_params["time_to"] = time_to
 
-        return url + urlencode(url_params)
+        return url + "?" + urlencode(url_params)
 
 
 class InfrahubBranchManager(InfraHubBranchManagerBase):