diff --git a/plugin.rb b/plugin.rb
index c6ced1a..178b241 100644
--- a/plugin.rb
+++ b/plugin.rb
@@ -556,4 +556,23 @@ after_initialize do
     end
   end
 
+  on(:user_removed_from_group) do |user, group|
+    assign_allowed_groups = SiteSetting.assign_allowed_on_groups.split('|').map(&:to_i)
+
+    if assign_allowed_groups.include?(group.id)
+      groups = GroupUser.where(user: user).pluck(:group_id)
+
+      if (groups & assign_allowed_groups).empty?
+        topics = Topic.joins(:_custom_fields)
+          .where(
+            'topic_custom_fields.name = ? AND topic_custom_fields.value = ?',
+            TopicAssigner::ASSIGNED_TO_ID, user.id.to_s
+          )
+
+        topics.each do |topic|
+          TopicAssigner.new(topic, Discourse.system_user).unassign
+        end
+      end
+    end
+  end
 end
diff --git a/spec/plugin_spec.rb b/spec/plugin_spec.rb
new file mode 100644
index 0000000..11fb716
--- /dev/null
+++ b/spec/plugin_spec.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'plugin' do
+  before { SiteSetting.assign_enabled = true }
+
+  describe 'events' do
+    describe 'on user_removed_from_group' do
+      before do
+        @topic = Fabricate(:post).topic
+        @user = Fabricate(:user)
+        @group_a = Fabricate(:group)
+        @group_a.add(@user)
+      end
+
+      it 'unassigns the user' do
+        SiteSetting.assign_allowed_on_groups = @group_a.id.to_s
+
+        TopicAssigner.new(@topic, Discourse.system_user).assign(@user)
+        @group_a.remove(@user)
+
+        custom_fields = @topic.reload.custom_fields
+        expect(custom_fields[TopicAssigner::ASSIGNED_TO_ID]).to be_nil
+        expect(custom_fields[TopicAssigner::ASSIGNED_BY_ID]).to be_nil
+      end
+
+      it "doesn't unassign the user if it still has access through another group" do
+        @group_b = Fabricate(:group)
+        @group_b.add(@user)
+        SiteSetting.assign_allowed_on_groups = [@group_a.id.to_s, @group_b.id.to_s].join('|')
+
+        TopicAssigner.new(@topic, Discourse.system_user).assign(@user)
+        @group_a.remove(@user)
+
+        custom_fields = @topic.reload.custom_fields
+        expect(custom_fields[TopicAssigner::ASSIGNED_TO_ID]).to eq(@user.id.to_s)
+        expect(custom_fields[TopicAssigner::ASSIGNED_BY_ID]).to eq(Discourse::SYSTEM_USER_ID.to_s)
+      end
+    end
+  end
+end