diff --git a/lib/diaspora_federation/entities/comment.rb b/lib/diaspora_federation/entities/comment.rb
index f271b1da..a81bbc27 100644
--- a/lib/diaspora_federation/entities/comment.rb
+++ b/lib/diaspora_federation/entities/comment.rb
@@ -24,6 +24,12 @@ class Comment < Entity
       #   The timestamp when the comment was edited
       #   @return [Time] edited time
       property :edited_at, :timestamp, optional: true
+
+      # @!attribute [r] guid
+      #   A random string of at least 16 chars pointing to a parent comment.
+      #   @see Validation::Rule::Guid
+      #   @return [String] parent comment guid
+      property :thread_parent_guid, :string, optional: true
     end
   end
 end
diff --git a/spec/lib/diaspora_federation/entities/comment_spec.rb b/spec/lib/diaspora_federation/entities/comment_spec.rb
index 2efb00da..c2ac875c 100644
--- a/spec/lib/diaspora_federation/entities/comment_spec.rb
+++ b/spec/lib/diaspora_federation/entities/comment_spec.rb
@@ -4,13 +4,15 @@ module DiasporaFederation
   describe Entities::Comment do
     let(:parent) { Fabricate(:post, author: bob) }
     let(:parent_entity) { Fabricate(:related_entity, author: bob.diaspora_id) }
+    let(:parent_comment) { Fabricate(:comment) }
     let(:data) {
       Fabricate
         .attributes_for(
           :comment_entity,
-          author:      alice.diaspora_id,
-          parent_guid: parent.guid,
-          parent:      parent_entity
+          author:             alice.diaspora_id,
+          parent_guid:        parent.guid,
+          parent:             parent_entity,
+          thread_parent_guid: parent_comment.guid
         ).tap {|hash| add_signatures(hash) }
     }
 
@@ -22,6 +24,7 @@ module DiasporaFederation
         <text>#{data[:text]}</text>
         <created_at>#{data[:created_at].utc.iso8601}</created_at>
         <edited_at>#{data[:edited_at].utc.iso8601}</edited_at>
+        <thread_parent_guid>#{parent_comment.guid}</thread_parent_guid>
         <author_signature>#{data[:author_signature]}</author_signature>
       </comment>
     XML
@@ -36,7 +39,8 @@ module DiasporaFederation
           "author_signature": "#{data[:author_signature]}",
           "text": "#{data[:text]}",
           "created_at": "#{data[:created_at].iso8601}",
-          "edited_at": "#{data[:edited_at].iso8601}"
+          "edited_at": "#{data[:edited_at].iso8601}",
+          "thread_parent_guid": "#{parent_comment.guid}"
         },
         "property_order": [
           "author",
@@ -44,7 +48,8 @@ module DiasporaFederation
           "parent_guid",
           "text",
           "created_at",
-          "edited_at"
+          "edited_at",
+          "thread_parent_guid"
         ]
       }
     JSON