Instonians is a sports club based in Belfast, Northern Ireland, that incorporates rugby union, men's and ladies' hockey and cricket sections. There is also a golf society that plays under the Instonians name.

Instonians Rugby Football, Cricket and Athletic Clubs merged on 1 April 1962, the combined sections being known as 'Instonians', and then on 31 May 1979 the title of the club was changed to Instonians Rugby Football and Cricket Club. Finally on 1 April 1988, the Instonians Men's Hockey Club joined the association when the title reverted to Instonians.

The club was closed to former pupils or staff of Royal Belfast Academical Institution, which regards itself as one of the leading schools in Belfast. Instonians was the last 'old boys' club in Ireland to finally open its membership to all in 1990.

The club for many years used the Royal Belfast Academical Institution grounds at Bladon Drive, Osborne Park and Orangefield, before moving to its own grounds at Shane Park in 1965. Shane Park was sold for redevelopment in 1999 and the club now groundshares at the Shaw's Bridge Sports Complex with Cooke.

Instonians Rugby Club started as a section within the Belfast Old Instonians Association. The original part of the modern-day club, it started play in the Ulster Junior League in season 1919–20, and produced play of sufficient quality to merit immediate election to the Ulster Senior League for the following season.

As of 2025 the 1st XV currently plays in the All Ireland League, Division 1B.

Instonians have won the Ulster Senior Cup twenty times, the Ulster Junior Cup four times and the Ulster Senior League seven times (with five shared titles).

31 Instonians rugby players have been capped by Ireland at full International level and the club has produced two British and Irish Lions captains.