diff --git a/lib/shared/dm-ops/dm-op-specs.js b/lib/shared/dm-ops/dm-op-specs.js
--- a/lib/shared/dm-ops/dm-op-specs.js
+++ b/lib/shared/dm-ops/dm-op-specs.js
@@ -3,6 +3,7 @@
 import { createSidebarSpec } from './create-sidebar-spec.js';
 import { createThreadSpec } from './create-thread-spec.js';
 import type { DMOperationSpec } from './dm-op-spec.js';
+import { sendReactionMessageSpec } from './send-reaction-message-spec.js';
 import { sendTextMessageSpec } from './send-text-message-spec.js';
 import { type DMOperationType, dmOperationTypes } from '../../types/dm-ops.js';
 
@@ -12,4 +13,5 @@
   [dmOperationTypes.CREATE_THREAD]: createThreadSpec,
   [dmOperationTypes.CREATE_SIDEBAR]: createSidebarSpec,
   [dmOperationTypes.SEND_TEXT_MESSAGE]: sendTextMessageSpec,
+  [dmOperationTypes.SEND_REACTION_MESSAGE]: sendReactionMessageSpec,
 });
diff --git a/lib/shared/dm-ops/send-reaction-message-spec.js b/lib/shared/dm-ops/send-reaction-message-spec.js
new file mode 100644
--- /dev/null
+++ b/lib/shared/dm-ops/send-reaction-message-spec.js
@@ -0,0 +1,54 @@
+// @flow
+
+import uuid from 'uuid';
+
+import type { DMOperationSpec } from './dm-op-spec.js';
+import type { DMSendReactionMessageOperation } from '../../types/dm-ops.js';
+import { messageTypes } from '../../types/message-types-enum.js';
+import { updateTypes } from '../../types/update-types-enum.js';
+import type { ClientUpdateInfo } from '../../types/update-types.js';
+
+const sendReactionMessageSpec: DMOperationSpec<DMSendReactionMessageOperation> =
+  Object.freeze({
+    processDMOperation: async (
+      dmOperation: DMSendReactionMessageOperation,
+      viewerID: string,
+    ) => {
+      const {
+        threadID,
+        creatorID,
+        time,
+        messageID,
+        targetMessageID,
+        reaction,
+        action,
+      } = dmOperation;
+      const reactionMessage = {
+        type: messageTypes.REACTION,
+        id: messageID,
+        threadID,
+        creatorID,
+        time,
+        targetMessageID,
+        reaction,
+        action,
+      };
+
+      const updateInfos: Array<ClientUpdateInfo> = [];
+      if (creatorID !== viewerID) {
+        updateInfos.push({
+          type: updateTypes.UPDATE_THREAD_READ_STATUS,
+          id: uuid.v4(),
+          time,
+          threadID,
+          unread: true,
+        });
+      }
+      return {
+        rawMessageInfos: [reactionMessage],
+        updateInfos,
+      };
+    },
+  });
+
+export { sendReactionMessageSpec };
diff --git a/lib/types/dm-ops.js b/lib/types/dm-ops.js
--- a/lib/types/dm-ops.js
+++ b/lib/types/dm-ops.js
@@ -16,6 +16,7 @@
   CREATE_THREAD: 'create_thread',
   CREATE_SIDEBAR: 'create_sidebar',
   SEND_TEXT_MESSAGE: 'send_text_message',
+  SEND_REACTION_MESSAGE: 'send_reaction_message',
 });
 export type DMOperationType = $Values<typeof dmOperationTypes>;
 
@@ -85,14 +86,38 @@
     text: t.String,
   });
 
+export type DMSendReactionMessageOperation = {
+  +type: 'send_reaction_message',
+  +threadID: string,
+  +creatorID: string,
+  +time: number,
+  +messageID: string,
+  +targetMessageID: string,
+  +reaction: string,
+  +action: 'add_reaction' | 'remove_reaction',
+};
+export const dmSendReactionMessageOperationValidator: TInterface<DMSendReactionMessageOperation> =
+  tShape<DMSendReactionMessageOperation>({
+    type: tString(dmOperationTypes.SEND_REACTION_MESSAGE),
+    threadID: t.String,
+    creatorID: tUserID,
+    time: t.Number,
+    messageID: t.String,
+    targetMessageID: t.String,
+    reaction: t.String,
+    action: t.enums.of(['add_reaction', 'remove_reaction']),
+  });
+
 export type DMOperation =
   | DMCreateThreadOperation
   | DMCreateSidebarOperation
-  | DMSendTextMessageOperation;
+  | DMSendTextMessageOperation
+  | DMSendReactionMessageOperation;
 export const dmOperationValidator: TUnion<DMOperation> = t.union([
   dmCreateThreadOperationValidator,
   dmCreateSidebarOperationValidator,
   dmSendTextMessageOperationValidator,
+  dmSendReactionMessageOperationValidator,
 ]);
 
 export type DMOperationResult = {