Fosnøyna or Fosnøy is an island in the municipality of Austrheim in Vestland county, Norway.

The 26.3-square-kilometre (10.2 sq mi) island makes up the majority of the land area of the municipality.

The island is located east of the Fedjefjorden and south of the Fensfjorden.

The highest point on the island is the 56-metre (184 ft) high Bredvikvarden.

The island is connected to the mainland road network by two bridges.