Osterfjorden is a fjord in Hordaland county, Norway. The fjord is one of three fjords surrounding the island of Osterøy. The fjord runs along the municipal border of Lindås and Osterøy municipalities. The fjord begins at the Romarheimsfjorden and flows to the west for 27 kilometres (17 mi) before ending near the village of Knarvik at the confluence of four fjords: Osterfjorden, Radfjorden (to the north), Sørfjorden (to the south), and Salhusfjorden (to the west).