The Tri Nations is contested annually by Australia, New Zealand and South Africa, and is considered by some to be the toughest competition in international rugby. The competition which began in 1996 is organized by SANZAR, a consortium formed by the three countries' rugby governing bodies – the Australian Rugby Union, the New Zealand Rugby Union and the South African Rugby Union.

In 2012, The Tri Nations could become the Four Nations if the inclusion of Argentina as the fourth team go ahead. Argentina has been invited to join but there are a number of conditions that they must fulfil.

The series is played on a home-and-away basis. Initially the three teams played each other twice, but since 2005 each team plays the others three times, except in the Rugby World Cup year of 2007 when the series reverted to a double round-robin. Since the inception of the series, the games played between Australia and New Zealand also go toward determining the winner of the Bledisloe Cup. The Freedom Cup is contested between New Zealand and South Africa, and the Mandela Challenge Plate between Australia and South Africa.