diff --git a/lib/selectors/chat-selectors.js b/lib/selectors/chat-selectors.js
--- a/lib/selectors/chat-selectors.js
+++ b/lib/selectors/chat-selectors.js
@@ -272,6 +272,7 @@
   endsCluster: boolean,
   +robotext: string,
   +threadCreatedFromMessage: ?ThreadInfo,
+  +reactions: $ReadOnlyMap<string, MessageReactionInfo>,
 };
 export type ChatMessageInfoItem =
   | RobotextChatMessageInfoItem
@@ -283,8 +284,14 @@
       +startsCluster: boolean,
       endsCluster: boolean,
       +threadCreatedFromMessage: ?ThreadInfo,
+      +reactions: $ReadOnlyMap<string, MessageReactionInfo>,
     };
 export type ChatMessageItem = { itemType: 'loader' } | ChatMessageInfoItem;
+export type MessageReactionInfo = {
+  +viewerReacted: boolean,
+  +users: $ReadOnlySet<string>,
+};
+
 const msInFiveMinutes = 5 * 60 * 1000;
 function createChatMessageItems(
   threadID: string,
@@ -293,6 +300,7 @@
   threadInfos: { +[id: string]: ThreadInfo },
   threadInfoFromSourceMessageID: { +[id: string]: ThreadInfo },
   additionalMessages: $ReadOnlyArray<MessageInfo>,
+  viewerID: string,
 ): ChatMessageItem[] {
   const thread = messageStore.threads[threadID];
 
@@ -304,6 +312,43 @@
       ? sortMessageInfoList([...threadMessageInfos, ...additionalMessages])
       : threadMessageInfos;
 
+  const targetMessageReactionsMap = new Map<string, Map<string, Set<string>>>();
+
+  // We need to iterate backwards to put the order of messages in chronological
+  // order, starting with the oldest. This avoids the scenario where the most
+  // recent message with the remove_reaction action may try to remove a user
+  // that hasn't been added to the usersSet, causing it to be skipped.
+  for (let i = messages.length - 1; i >= 0; i--) {
+    const messageInfo = messages[i];
+    if (messageInfo.type !== messageTypes.REACTION) {
+      continue;
+    }
+
+    if (!targetMessageReactionsMap.has(messageInfo.targetMessageID)) {
+      const reactsMap = new Map<string, Set<string>>();
+      targetMessageReactionsMap.set(messageInfo.targetMessageID, reactsMap);
+    }
+
+    const messageReactsMap = targetMessageReactionsMap.get(
+      messageInfo.targetMessageID,
+    );
+    invariant(messageReactsMap, 'messageReactsInfo should be set');
+
+    if (!messageReactsMap.has(messageInfo.reaction)) {
+      const usersSet = new Set<string>();
+      messageReactsMap.set(messageInfo.reaction, usersSet);
+    }
+
+    const messageReactionUsersSet = messageReactsMap.get(messageInfo.reaction);
+    invariant(messageReactionUsersSet, 'messageReactionUsersSet should be set');
+
+    if (messageInfo.action === 'add_reaction') {
+      messageReactionUsersSet.add(messageInfo.creator.id);
+    } else {
+      messageReactionUsersSet.delete(messageInfo.creator.id);
+    }
+  }
+
   const chatMessageItems = [];
   let lastMessageInfo = null;
   for (let i = messages.length - 1; i >= 0; i--) {
@@ -346,6 +391,43 @@
       messageInfo.id && threadInfos[threadID]?.type !== threadTypes.SIDEBAR
         ? threadInfoFromSourceMessageID[messageInfo.id]
         : undefined;
+
+    const renderedReactions: $ReadOnlyMap<
+      string,
+      MessageReactionInfo,
+    > = (() => {
+      const result = new Map<string, MessageReactionInfo>();
+
+      let messageReactsMap;
+      if (originalMessageInfo.id) {
+        messageReactsMap = targetMessageReactionsMap.get(
+          originalMessageInfo.id,
+        );
+      }
+
+      if (!messageReactsMap) {
+        return result;
+      }
+
+      for (const reaction of messageReactsMap.keys()) {
+        const reactionUsersSet = messageReactsMap.get(reaction);
+        invariant(reactionUsersSet, 'reactionUsersSet should be set');
+
+        if (reactionUsersSet.size === 0) {
+          continue;
+        }
+
+        const messageReactionInfo = {
+          users: reactionUsersSet,
+          viewerReacted: reactionUsersSet.has(viewerID),
+        };
+
+        result.set(reaction, messageReactionInfo);
+      }
+
+      return result;
+    })();
+
     if (isComposableMessageType(originalMessageInfo.type)) {
       // We use these invariants instead of just checking the messageInfo.type
       // directly in the conditional above so that isComposableMessageType can
@@ -366,6 +448,7 @@
         startsCluster,
         endsCluster: false,
         threadCreatedFromMessage,
+        reactions: renderedReactions,
       });
     } else {
       invariant(
@@ -386,6 +469,7 @@
         endsCluster: false,
         threadCreatedFromMessage,
         robotext,
+        reactions: renderedReactions,
       });
     }
     lastMessageInfo = originalMessageInfo;
@@ -412,13 +496,16 @@
     messageInfoSelector,
     threadInfoSelector,
     threadInfoFromSourceMessageIDSelector,
+    (state: BaseAppState<*>) =>
+      state.currentUserInfo && state.currentUserInfo.id,
     (
       messageStore: MessageStore,
       messageInfos: { +[id: string]: ?MessageInfo },
       threadInfos: { +[id: string]: ThreadInfo },
       threadInfoFromSourceMessageID: { +[id: string]: ThreadInfo },
+      viewerID: ?string,
     ): ?(ChatMessageItem[]) => {
-      if (!threadID) {
+      if (!threadID || !viewerID) {
         return null;
       }
       return createChatMessageItems(
@@ -428,6 +515,7 @@
         threadInfos,
         threadInfoFromSourceMessageID,
         additionalMessages,
+        viewerID,
       );
     },
   );