diff --git a/python/ray/tune/suggest/hyperopt.py b/python/ray/tune/suggest/hyperopt.py
index f129ee129..d3424dafb 100644
--- a/python/ray/tune/suggest/hyperopt.py
+++ b/python/ray/tune/suggest/hyperopt.py
@@ -304,7 +304,7 @@ class HyperOptSearch(Searcher):
             self.set_state(trials_object)
 
     @staticmethod
-    def convert_search_space(spec: Dict) -> Dict:
+    def convert_search_space(spec: Dict, prefix: str = "") -> Dict:
         spec = copy.deepcopy(spec)
         resolved_vars, domain_vars, grid_vars = parse_spec_vars(spec)
 
@@ -361,7 +361,17 @@ class HyperOptSearch(Searcher):
                     return hpo.hp.randint(par, domain.upper)
             elif isinstance(domain, Categorical):
                 if isinstance(sampler, Uniform):
-                    return hpo.hp.choice(par, domain.categories)
+                    return hpo.hp.choice(par, [
+                        HyperOptSearch.convert_search_space(
+                            category, prefix=par)
+                        if isinstance(category, dict) else
+                        HyperOptSearch.convert_search_space(
+                            dict(enumerate(category)), prefix=f"{par}/{i}")
+                        if isinstance(category, list) else resolve_value(
+                            f"{par}/{i}", category)
+                        if isinstance(category, Domain) else category
+                        for i, category in enumerate(domain.categories)
+                    ])
 
             raise ValueError("HyperOpt does not support parameters of type "
                              "`{}` with samplers of type `{}`".format(
@@ -369,7 +379,8 @@ class HyperOptSearch(Searcher):
                                  type(domain.sampler).__name__))
 
         for path, domain in domain_vars:
-            par = "/".join(path)
+            par = "/".join(
+                [str(p) for p in ((prefix, ) + path if prefix else path)])
             value = resolve_value(par, domain)
             assign_value(spec, path, value)
 
diff --git a/python/ray/tune/tests/test_sample.py b/python/ray/tune/tests/test_sample.py
index ca61627fd..3cae66ed5 100644
--- a/python/ray/tune/tests/test_sample.py
+++ b/python/ray/tune/tests/test_sample.py
@@ -408,6 +408,52 @@ class SearchSpaceTest(unittest.TestCase):
         trial = analysis.trials[0]
         assert trial.config["a"] in [2, 3, 4]
 
+    def testConvertHyperOptNested(self):
+        from ray.tune.suggest.hyperopt import HyperOptSearch
+
+        config = {
+            "a": 1,
+            "dict_nested": tune.sample.Categorical([{
+                "a": tune.sample.Categorical(["M", "N"]),
+                "b": tune.sample.Categorical(["O", "P"])
+            }]).uniform(),
+            "list_nested": tune.sample.Categorical([
+                [
+                    tune.sample.Categorical(["M", "N"]),
+                    tune.sample.Categorical(["O", "P"])
+                ],
+                [
+                    tune.sample.Categorical(["Q", "R"]),
+                    tune.sample.Categorical(["S", "T"])
+                ],
+            ]).uniform(),
+            "domain_nested": tune.sample.Categorical([
+                tune.sample.Categorical(["M", "N"]),
+                tune.sample.Categorical(["O", "P"])
+            ]).uniform(),
+        }
+
+        searcher = HyperOptSearch(metric="a", mode="max")
+        analysis = tune.run(
+            _mock_objective,
+            config=config,
+            search_alg=searcher,
+            num_samples=10)
+
+        for trial in analysis.trials:
+            config = trial.config
+
+            self.assertIn(config["dict_nested"]["a"], ["M", "N"])
+            self.assertIn(config["dict_nested"]["b"], ["O", "P"])
+
+            if config["list_nested"][0] in ["M", "N"]:
+                self.assertIn(config["list_nested"][1], ["O", "P"])
+            else:
+                self.assertIn(config["list_nested"][0], ["Q", "R"])
+                self.assertIn(config["list_nested"][1], ["S", "T"])
+
+            self.assertIn(config["domain_nested"], ["M", "N", "O", "P"])
+
     def testConvertNevergrad(self):
         from ray.tune.suggest.nevergrad import NevergradSearch
         import nevergrad as ng
diff --git a/python/ray/tune/utils/util.py b/python/ray/tune/utils/util.py
index 45b8fca09..e77449a76 100644
--- a/python/ray/tune/utils/util.py
+++ b/python/ray/tune/utils/util.py
@@ -251,7 +251,7 @@ def flatten_dict(dt, delimiter="/", prevent_delimiter=False):
                             "Found delimiter `{}` in key when trying to "
                             "flatten array. Please avoid using the delimiter "
                             "in your specification.")
-                    add[delimiter.join([key, subkey])] = v
+                    add[delimiter.join([key, str(subkey)])] = v
                 remove.append(key)
         dt.update(add)
         for k in remove: