diff --git a/rest_framework/fields.py b/rest_framework/fields.py
index deca9d90dc..a2b0f3bf18 100644
--- a/rest_framework/fields.py
+++ b/rest_framework/fields.py
@@ -1204,6 +1204,8 @@ def to_representation(self, value):
         )
 
         if output_format.lower() == ISO_8601:
+            if isinstance(value, six.string_types):
+                value = datetime.datetime.strptime(value, '%H:%M:%S').time()
             return value.isoformat()
         return value.strftime(output_format)
 
diff --git a/tests/test_fields.py b/tests/test_fields.py
index 4b3a36f031..43441c2e7c 100644
--- a/tests/test_fields.py
+++ b/tests/test_fields.py
@@ -1032,6 +1032,7 @@ class TestTimeField(FieldValues):
     }
     outputs = {
         datetime.time(13, 00): '13:00:00',
+        '00:00:00': '00:00:00',
         None: None,
         '': None,
     }