The Royal Institute of International Affairs, also known as Chatham House, is a British think tank based in London, England. Its stated mission is "to help governments and societies build a sustainably secure, prosperous, and just world." Conversely, it has been praised for providing a safe space for speakers and encouraging openness.

The Royal Institute of International Affairs has its headquarters in central London at 10 St James's Square, which is known as Chatham House. It is a Grade I listed 18th-century building that was designed in part by Henry Flitcroft and was occupied by three British prime ministers, including William Pitt, 1st Earl of Chatham, whose name became associated with the house. Canadian philanthropists Lieutenant-Colonel Reuben Wells Leonard and Kate Rowlands Leonard purchased the property in 1923 and then donated the building to the fledgling institute as its headquarters. As a result, the Chatham House name is used as a metonym for the institute as a whole.

Chatham House accepts individual members, as well as members from corporations, academic institutions, embassies, and NGOs. The institute has also faced scrutiny over perceived elitism, limited funding transparency, alignment with interventionist foreign policy positions, and its early links to British imperialism. It is the originator of the Chatham House Rule.

Chatham House is the origin of the non-attribution rule known as the Chatham House Rule, which provides that a participant in a meeting may discuss the content of this meeting in the outside world, but may not discuss who attended nor identify what a specific individual said. The Chatham House Rule evolved to facilitate frank and honest discussion on controversial or unpopular issues by speakers who may not have otherwise had the appropriate forum to speak freely. Most meetings at Chatham House are held on the record rather than under the Chatham House Rule.

Chatham House is structured around thirteen research programmes, comprising six thematic programmes and seven regional programmes.

Thematic programmes:

The latter four programmes are grouped together under the umbrella of the Global Governance and Security Centre, which looks at evolving challenges to the international norms, values and institutions of the current global order.  

The Environment and Society Centre also houses the Sustainability Accelerator, an initiative that aims to combine the best of evidence-based policymaking with the experimental approach of the entrepreneurship community to reach sustainability solutions.