diff --git a/public/openapi/read.yaml b/public/openapi/read.yaml
index f0cb992675..995a610c9b 100644
--- a/public/openapi/read.yaml
+++ b/public/openapi/read.yaml
@@ -3542,6 +3542,9 @@ paths:
                         type: number
                       viewcount:
                         type: number
+                      posterCount:
+                        type: number
+                        description: The number of unique users who made a post in this topic
                       mainPid:
                         type: number
                         description: The post id of the first post in this topic (also called the
diff --git a/src/topics/index.js b/src/topics/index.js
index a80be8b1cf..74e0395e85 100644
--- a/src/topics/index.js
+++ b/src/topics/index.js
@@ -146,6 +146,7 @@ Topics.getTopicWithPosts = async function (topicData, set, uid, start, stop, rev
 		deleter,
 		merger,
 		related,
+		posterCount,
 	] = await Promise.all([
 		getMainPostAndReplies(topicData, set, uid, start, stop, reverse),
 		categories.getCategoryData(topicData.cid),
@@ -157,6 +158,7 @@ Topics.getTopicWithPosts = async function (topicData, set, uid, start, stop, rev
 		getDeleter(topicData),
 		getMerger(topicData),
 		getRelated(topicData, uid),
+		db.sortedSetCard('tid:' + topicData.tid + ':posters'),
 	]);
 
 	topicData.posts = posts;
@@ -179,6 +181,7 @@ Topics.getTopicWithPosts = async function (topicData, set, uid, start, stop, rev
 		topicData.mergedTimestampISO = utils.toISOString(topicData.mergedTimestamp);
 	}
 	topicData.related = related || [];
+	topicData.posterCount = posterCount;
 	topicData.unreplied = topicData.postcount === 1;
 	topicData.icons = [];