The Royal Institute of British Architects (RIBA) is a professional body for architects primarily in the United Kingdom, but also internationally, founded for the advancement of architecture under its royal charter granted in 1837, three supplemental charters and a new charter granted in 1971.

Founded as the Institute of British Architects in London in 1834, the RIBA retains a central London headquarters at 66 Portland Place as well as a network of regional offices. Its members played a leading part in promotion of architectural education in the United Kingdom; the RIBA Library, also established in 1834, is one of the three largest architectural libraries in the world and the largest in Europe. The RIBA also played a prominent role in the development of UK architects' registration bodies.

The institute administers some of the oldest architectural awards in the world, including RIBA President's Medals Students Award, the Royal Gold Medal, and the Stirling Prize. It also manages RIBA Competitions, organising architectural and other design-related competitions.

The RIBA was historically a male-dominated body, first admitting women members in 1898, and appointing its first female president in 2009. Sometimes perceived as a London-centric organisation, it has also been accused of lacking transparency.