diff --git a/lib/types/message-types.js b/lib/types/message-types.js
--- a/lib/types/message-types.js
+++ b/lib/types/message-types.js
@@ -543,6 +543,12 @@
       +mediaMessageContents: $ReadOnlyArray<MediaMessageServerDBContent>,
     };
 
+export type SendReactionMessageRequest = {
+  +threadID: string,
+  +targetMessageID: string,
+  +reaction: string | null,
+};
+
 // Used for the message info included in log-in type actions
 export type GenericMessagesResult = {
   +messageInfos: RawMessageInfo[],