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
@@ -125,6 +125,7 @@
   SIDEBAR_SOURCE: 17,
   // Appears in the newly created sidebar
   CREATE_SIDEBAR: 18,
+  REACTION: 19,
 });
 export type MessageType = $Values<typeof messageTypes>;
 export function assertMessageType(ourMessageType: number): MessageType {
@@ -147,7 +148,8 @@
       ourMessageType === 15 ||
       ourMessageType === 16 ||
       ourMessageType === 17 ||
-      ourMessageType === 18,
+      ourMessageType === 18 ||
+      ourMessageType === 19,
     'number is not MessageType enum',
   );
   return ourMessageType;
@@ -217,6 +219,16 @@
   +time: number,
   +sourceMessage?: RawComposableMessageInfo | RawRobotextMessageInfo,
 };
+
+export type ReactionMessageData = {
+  +type: 19,
+  +threadID: string,
+  +creatorID: string,
+  +time: number,
+  +targetMessageID: string,
+  +reaction: string | null,
+};
+
 export type MessageData =
   | TextMessageData
   | CreateThreadMessageData
@@ -235,7 +247,8 @@
   | MediaMessageData
   | UpdateRelationshipMessageData
   | SidebarSourceMessageData
-  | CreateSidebarMessageData;
+  | CreateSidebarMessageData
+  | ReactionMessageData;
 
 export type MultimediaMessageData = ImagesMessageData | MediaMessageData;
 
@@ -266,10 +279,17 @@
   ...SidebarSourceMessageData,
   id: string,
 };
+
+export type RawReactionMessageInfo = {
+  ...ReactionMessageData,
+  id: string,
+};
+
 export type RawMessageInfo =
   | RawComposableMessageInfo
   | RawRobotextMessageInfo
-  | RawSidebarSourceMessageInfo;
+  | RawSidebarSourceMessageInfo
+  | RawReactionMessageInfo;
 
 export type LocallyComposedMessageInfo =
   | ({