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
@@ -106,7 +106,10 @@
         if instance is None:
             instance = parse_url(url).host
         super().__init__(
-            scheduler=scheduler, url=url, instance=instance, credentials=credentials,
+            scheduler=scheduler,
+            url=url.rstrip("/"),
+            instance=instance,
+            credentials=credentials,
         )
         self.incremental = incremental
         self.last_page: Optional[str] = None
@@ -161,7 +164,7 @@
         }
         if id_after is not None:
             parameters["id_after"] = str(id_after)
-        return f"{self.url}projects?{urlencode(parameters)}"
+        return f"{self.url}/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"
 
 
+@pytest.mark.parametrize("url", [api_url("gitlab").rstrip("/"), api_url("gitlab"),])
+def test_lister_gitlab_url_computation(url, swh_scheduler):
+    lister = GitLabLister(scheduler=swh_scheduler, url=url)
+    assert not lister.url.endswith("/")
+
+    page_url = lister.page_url()
+    # ensure the generated url contains the separated /
+    assert page_url.startswith(f"{lister.url}/projects")
+
+
 @pytest.mark.parametrize(
     "url,expected_result",
     [