diff --git a/app/assets/javascripts/discourse/controllers/topic.js.es6 b/app/assets/javascripts/discourse/controllers/topic.js.es6
index 01161633777..50fdf050ad5 100644
--- a/app/assets/javascripts/discourse/controllers/topic.js.es6
+++ b/app/assets/javascripts/discourse/controllers/topic.js.es6
@@ -989,12 +989,30 @@ export default Ember.Controller.extend(BufferedContent, {
       : undefined;
   },
 
-  @computed("selectedPostsCount", "model.postStream.stream.length")
-  selectedAllPosts(selectedPostsCount, postsCount) {
-    return selectedPostsCount >= postsCount;
+  @computed(
+    "selectedPostsCount",
+    "model.postStream.isMegaTopic",
+    "model.postStream.stream.length",
+    "model.posts_count"
+  )
+  selectedAllPosts(
+    selectedPostsCount,
+    isMegaTopic,
+    postsCount,
+    topicPostsCount
+  ) {
+    if (isMegaTopic) {
+      return selectedPostsCount >= topicPostsCount;
+    } else {
+      return selectedPostsCount >= postsCount;
+    }
+  },
+
+  @computed("selectedAllPosts", "model.postStream.isMegaTopic")
+  canSelectAll(selectedAllPosts, isMegaTopic) {
+    return isMegaTopic ? false : !selectedAllPosts;
   },
 
-  canSelectAll: Ember.computed.not("selectedAllPosts"),
   canDeselectAll: Ember.computed.alias("selectedAllPosts"),
 
   @computed(
diff --git a/app/assets/javascripts/discourse/templates/selected-posts.hbs b/app/assets/javascripts/discourse/templates/selected-posts.hbs
index d4ac87ead3b..87bb29fe5df 100644
--- a/app/assets/javascripts/discourse/templates/selected-posts.hbs
+++ b/app/assets/javascripts/discourse/templates/selected-posts.hbs
@@ -1,7 +1,7 @@
 <p>{{count-i18n key="topic.multi_select.description" count=selectedPostsCount}}</p>
 
 {{#if canSelectAll}}
-  <p><a href {{action "selectAll"}}>{{i18n 'topic.multi_select.select_all'}}</a></p>
+  <p><a class="select-all" href {{action "selectAll"}}>{{i18n 'topic.multi_select.select_all'}}</a></p>
 {{/if}}
 
 {{#if canDeselectAll}}
diff --git a/test/javascripts/acceptance/topic-test.js.es6 b/test/javascripts/acceptance/topic-test.js.es6
index 57cff8e85b1..1ac08cc3199 100644
--- a/test/javascripts/acceptance/topic-test.js.es6
+++ b/test/javascripts/acceptance/topic-test.js.es6
@@ -252,6 +252,11 @@ QUnit.test("selecting posts", async assert => {
     "it should show the multi select menu"
   );
 
+  assert.ok(
+    exists(".select-all"),
+    "it should allow users to select all the posts"
+  );
+
   await click(".toggle-admin-menu");
 
   assert.ok(
diff --git a/test/javascripts/controllers/topic-test.js.es6 b/test/javascripts/controllers/topic-test.js.es6
index 0e3c4e72216..968c8d9f777 100644
--- a/test/javascripts/controllers/topic-test.js.es6
+++ b/test/javascripts/controllers/topic-test.js.es6
@@ -123,6 +123,16 @@ QUnit.test("selectedAllPosts", function(assert) {
     controller.get("selectedAllPosts"),
     "all posts (including filtered posts) are selected"
   );
+
+  model.setProperties({
+    "postStream.isMegaTopic": true,
+    posts_count: 1
+  });
+
+  assert.ok(
+    controller.get("selectedAllPosts"),
+    "it uses the topic's post count for megatopics"
+  );
 });
 
 QUnit.test("selectedPostsUsername", function(assert) {