Pheasant Island  is a river island in the Bidasoa river. The island is a condominium established by the Treaty of the Pyrenees in 1659, under joint sovereignty of Spain and France, and is administered by Irun (in Gipuzkoa, Spain) and Hendaye (in the Pyrénées-Atlantiques, France), which control the island for alternating periods of six months. The island has an area of 6,820 m² and has been artificially protected from the currents of the river.

The island was the site of many royal meetings, but the most important historical event that took place on the island was the signing of the Treaty of the Pyrenees after a series of 24 conferences between Luis de Haro and Cardinal Mazarin in 1659 following the end of the Thirty Years' War. A monolith was built in the centre of the island to commemorate this event.