diff --git a/src/types/events/base-events.ts b/src/types/events/base-events.ts
index fdadc54be..290605041 100644
--- a/src/types/events/base-events.ts
+++ b/src/types/events/base-events.ts
@@ -611,15 +611,31 @@ export interface PinRemovedEvent extends StringIndexed {
   event_ts: string;
 }
 
+interface ReactionMessageItem {
+  type: 'message';
+  channel: string;
+  ts: string;
+}
+
+interface ReactionFileItem {
+  type: 'file';
+  file: string;
+}
+
+// This type is deprecated.
+// See https://api.slack.com/changelog/2018-05-file-threads-soon-tread
+interface ReactionFileCommentItem {
+  type: 'file_comment';
+  file_comment: string;
+  file: string;
+}
+
 export interface ReactionAddedEvent extends StringIndexed {
   type: 'reaction_added';
   user: string;
   reaction: string;
   item_user: string;
-  // TODO: incomplete, should be message | file | file comment (deprecated)
-  // https://api.slack.com/events/reaction_added
-  item: {
-  };
+  item: ReactionMessageItem | ReactionFileItem | ReactionFileCommentItem;
   event_ts: string;
 }