diff --git a/lib/active_model_serializers/adapter/json_api/deserialization.rb b/lib/active_model_serializers/adapter/json_api/deserialization.rb
index d846cacc4..0e8a203ab 100644
--- a/lib/active_model_serializers/adapter/json_api/deserialization.rb
+++ b/lib/active_model_serializers/adapter/json_api/deserialization.rb
@@ -189,7 +189,7 @@ def parse_relationship(assoc_name, assoc_data, options)
 
           polymorphic = (options[:polymorphic] || []).include?(assoc_name.to_sym)
           if polymorphic
-            hash["#{prefix_key}_type".to_sym] = assoc_data.present? ? assoc_data['type'].classify : nil
+            hash["#{prefix_key}_type".to_sym] = assoc_data.present? ? assoc_data['type'].underscore.classify : nil
           end
 
           hash