diff --git a/app/models/topic.rb b/app/models/topic.rb
index b8be34167ee..6b01d9e4535 100644
--- a/app/models/topic.rb
+++ b/app/models/topic.rb
@@ -662,12 +662,26 @@ SQL
 
       if self.category_id != new_category.id
         self.update!(category_id: new_category.id)
-        Category.where(id: old_category.id).update_all("topic_count = topic_count - 1") if old_category
+
+        if old_category
+          Category
+            .where(id: old_category.id)
+            .update_all("topic_count = topic_count - 1")
+        end
 
         # when a topic changes category we may have to start watching it
         # if we happen to have read state for it
         CategoryUser.auto_watch(category_id: new_category.id, topic_id: self.id)
         CategoryUser.auto_track(category_id: new_category.id, topic_id: self.id)
+
+        post = self.ordered_posts.first
+
+        if post
+          PostAlerter.new.notify_post_users(
+            post,
+            [post.user, post.last_editor].uniq
+          )
+        end
       end
 
       Category.where(id: new_category.id).update_all("topic_count = topic_count + 1")
diff --git a/spec/fabricators/topic_fabricator.rb b/spec/fabricators/topic_fabricator.rb
index 1fd06233cdd..b3efbb98e3d 100644
--- a/spec/fabricators/topic_fabricator.rb
+++ b/spec/fabricators/topic_fabricator.rb
@@ -17,7 +17,6 @@ Fabricator(:banner_topic, from: :topic) do
 end
 
 Fabricator(:private_message_topic, from: :topic) do
-  user
   category_id { nil }
   title { sequence(:title) { |i| "This is a private message #{i}" } }
   archetype "private_message"
diff --git a/spec/models/topic_spec.rb b/spec/models/topic_spec.rb
index 6a17b19bec7..aed6e653ddc 100644
--- a/spec/models/topic_spec.rb
+++ b/spec/models/topic_spec.rb
@@ -1191,11 +1191,8 @@ describe Topic do
         category.reload
       end
 
-      it 'increases the topic_count' do
-        expect(category.topic_count).to eq(1)
-      end
-
       it "doesn't change the topic_count when the value doesn't change" do
+        expect(category.topic_count).to eq(1)
         expect { topic.change_category_to_id(category.id); category.reload }.not_to change(category, :topic_count)
       end
 
@@ -1215,6 +1212,29 @@ describe Topic do
           expect(category.reload.topic_count).to eq(0)
         end
 
+        describe 'user that watching the new category' do
+          it 'should generate the notification for the topic' do
+            topic.posts << Fabricate(:post)
+
+            CategoryUser.set_notification_level_for_category(
+              user,
+              CategoryUser::notification_levels[:watching],
+              new_category.id
+            )
+
+            expect do
+              topic.change_category_to_id(new_category.id)
+            end.to change { Notification.count }.by(1)
+
+            notification = Notification.last
+
+            expect(notification.notification_type).to eq(Notification.types[:posted])
+            expect(notification.topic_id).to eq(topic.id)
+            expect(notification.user_id).to eq(user.id)
+            expect(notification.post_number).to eq(1)
+          end
+        end
+
         describe 'when new category is set to auto close by default' do
           before do
             new_category.update!(auto_close_hours: 5)