diff --git a/connexion/spec.py b/connexion/spec.py
index be5d653e5..9f4603953 100644
--- a/connexion/spec.py
+++ b/connexion/spec.py
@@ -269,7 +269,7 @@ def base_path(self):
 
     @base_path.setter
     def base_path(self, base_path):
-        base_path = canonical_base_path(base_path)
+        base_path = "/" + canonical_base_path(base_path).lstrip("/")
         self._raw_spec["basePath"] = base_path
         self._spec["basePath"] = base_path
 
diff --git a/tests/api/conftest.py b/tests/api/conftest.py
index bfde3ed03..e428c4426 100644
--- a/tests/api/conftest.py
+++ b/tests/api/conftest.py
@@ -24,11 +24,22 @@ def simple_openapi_app(app_class):
 
 
 @pytest.fixture(scope="session")
-def swagger_ui_app(app_class):
+def swagger_ui_app(spec, app_class):
     return build_app_from_fixture(
         "simple",
         app_class=app_class,
-        spec_file=OPENAPI3_SPEC,
+        spec_file=spec,
+        validate_responses=True,
+        swagger_ui_options=SwaggerUIOptions(spec_path="/spec.json"),
+    )
+
+
+@pytest.fixture(scope="session")
+def swagger_ui_basepath_app(app_class):
+    return build_app_from_fixture(
+        "simple",
+        app_class=app_class,
+        spec_file="basepath-slash.yaml",
         validate_responses=True,
         swagger_ui_options=SwaggerUIOptions(spec_path="/spec.json"),
     )
diff --git a/tests/api/test_swagger_ui.py b/tests/api/test_swagger_ui.py
index 1d787c65a..1d40e6381 100644
--- a/tests/api/test_swagger_ui.py
+++ b/tests/api/test_swagger_ui.py
@@ -1,4 +1,17 @@
+from connexion.spec import Specification
+
+
 def test_simple(swagger_ui_app):
     app_client = swagger_ui_app.test_client()
     response = app_client.get("/v1.0/spec.json")
     assert response.status_code == 200
+    # Load the spec into Connexion to validate it
+    Specification.from_dict(response.json())
+
+
+def test_basepath(swagger_ui_basepath_app):
+    app_client = swagger_ui_basepath_app.test_client()
+    response = app_client.get("/spec.json")
+    assert response.status_code == 200
+    # Load the spec into Connexion to validate it
+    Specification.from_dict(response.json())