diff --git a/app/contracts/messages/update_contract.rb b/app/contracts/messages/update_contract.rb
index d8668405b38..101ac13a810 100644
--- a/app/contracts/messages/update_contract.rb
+++ b/app/contracts/messages/update_contract.rb
@@ -31,5 +31,20 @@
 # TODO: This is but a stub
 module Messages
   class UpdateContract < BaseContract
+    validate :moving_message_to_another_forum
+
+    private
+
+    def moving_message_to_another_forum
+      return if !model.forum_id_changed?
+      return if model.forum_id_was.nil?
+
+      old_forum = Forum.find_by(id: model.forum_id_was)
+      return if old_forum.nil?
+
+      return if old_forum.project_id == model.forum.project_id
+
+      errors.add(:forum_id, :cannot_move_message_to_forum_of_different_project)
+    end
   end
 end
diff --git a/config/locales/en.yml b/config/locales/en.yml
index a39ea0e44c8..ab6c149476a 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -1740,6 +1740,8 @@ en:
               not_unique: "is already in use. Please select another name."
         meeting:
           error_conflict: "Unable to save because the meeting was updated by someone else in the meantime. Please reload the page."
+        message:
+          cannot_move_message_to_forum_of_different_project: "A message cannot be moved to a forum of a different project."
         notifications:
           at_least_one_channel: "At least one channel for sending notifications needs to be specified."
           attributes:
diff --git a/spec/contracts/messages/update_contract_spec.rb b/spec/contracts/messages/update_contract_spec.rb
index 32ea07b9d78..e437ff847c6 100644
--- a/spec/contracts/messages/update_contract_spec.rb
+++ b/spec/contracts/messages/update_contract_spec.rb
@@ -53,4 +53,37 @@ RSpec.describe Messages::UpdateContract do
       end
     end
   end
+
+  context "when moving a message to another forum" do
+    let(:project) { create(:project) }
+    let(:forum) { create(:forum, project: project) }
+    let(:other_forum) { create(:forum, project: project) }
+    let(:message) { create(:message, forum: forum) }
+    let(:forum_in_other_project) { create(:forum) }
+
+    let(:current_user) { create(:user, member_with_permissions: { project => [:edit_messages] }) }
+
+    subject(:contract) { described_class.new(message, current_user) }
+
+    context "when moving the message to another forum in the same project" do
+      before do
+        message.forum = other_forum
+      end
+
+      it "is valid" do
+        expect(contract).to be_valid
+      end
+    end
+
+    context "when moving the message to a forum in another project" do
+      before do
+        message.forum = forum_in_other_project
+      end
+
+      it "is invalid" do
+        expect(contract).not_to be_valid
+        expect(contract.errors[:forum_id]).to include("A message cannot be moved to a forum of a different project.")
+      end
+    end
+  end
 end