The Royal Institute of International Affairs, commonly known as Chatham House, is an independent international affairs think tank and membership foundation. Its mission is to help governments and societies build a sustainably secure, prosperous and just world. It conducts research into areas including environment and society, global economy and finance, global health security, international law, and international security.

The idea of an Anglo-American Institute of foreign affairs to study international problems with a view to preventing future wars was conceived by Lionel Curtis during the Paris Peace Conference of 1919. The British part of this, the British Institute of International Affairs, was founded separately in London in July 1920 and received its Royal Charter in 1926 to become The Royal Institute of International Affairs. The American delegates developed the Council on Foreign Relations in New York as a sister institute. Both are now among the world's leading international affairs think tanks.