diff --git a/swh/lister/gitlab/lister.py b/swh/lister/gitlab/lister.py
--- a/swh/lister/gitlab/lister.py
+++ b/swh/lister/gitlab/lister.py
@@ -103,6 +103,10 @@
         credentials: Optional[CredentialsType] = None,
         incremental: bool = False,
     ):
+        # to ensure urljoin will produce valid Sources URL
+        if not url.endswith("/"):
+            url += "/"
+
         if instance is None:
             instance = parse_url(url).host
         super().__init__(
@@ -161,7 +165,7 @@
         }
         if id_after is not None:
             parameters["id_after"] = str(id_after)
-        return f"{self.url}projects?{urlencode(parameters)}"
+        return f"{self.url.rstrip('/')}/projects?{urlencode(parameters)}"
 
     def get_pages(self) -> Iterator[PageResult]:
         next_page: Optional[str]
diff --git a/swh/lister/gitlab/tests/test_lister.py b/swh/lister/gitlab/tests/test_lister.py
--- a/swh/lister/gitlab/tests/test_lister.py
+++ b/swh/lister/gitlab/tests/test_lister.py
@@ -216,6 +216,16 @@
     assert lister.session.headers["Authorization"] == "Bearer api-token"
 
 
+def test_lister_gitlab_url_computation(swh_scheduler):
+    instance = "gitlab"
+    url = api_url(instance).rstrip("/")
+    assert not url.endswith("/")
+
+    lister = GitLabLister(scheduler=swh_scheduler, url=url)
+    assert lister.url == api_url(instance)
+    assert lister.url.endswith("/")
+
+
 @pytest.mark.parametrize(
     "url,expected_result",
     [