or is an island in Troms og Finnmark county, Norway, Europe. With an area of , it is the second largest island in Norway (outside of the Svalbard archipelago). It has a wild, mountainous outer (western) side facing the Atlantic, and a mild and lush inner (eastern) side. The island is located within Senja Municipality, which was established on 1 January 2020. The island of Senja had 7,864 inhabitants as of 1 January 2017. Most of the residents live along the eastern coast of the island, with Silsand being the largest urban area on the island. The fishing village of Gryllefjord on the west coast has a summer-only ferry connection to the nearby island of Andøya: the Andenes–Gryllefjord Ferry.
The island sits northeast of the Vesterålen archipelago, surrounded by the Norwegian Sea to the northwest, the Malangen fjord to the northeast, the Gisundet strait to the east, the Solbergfjorden to the southeast, the Vågsfjorden to the south, and the Andfjorden to the west. Ånderdalen National Park is located in the southern part of the island.
The Old Norse form of the name is believed to have been Senja or perhaps Sændja. The meaning of the name is unknown, but it might be related to the verb sundra, which means to "tear" or "split apart", possibly because the west coast of the island is torn and split by numerous small fjords. It might also be derived from a Proto-Norse form of the word Sandijōn, meaning "(area) of sand" or "sandy island".
The island of Senja is located along the Troms og Finnmark county coastline with Finnsnes as the closest town. Senja is connected to the mainland by the Gisund Bridge. The municipalities located on Senja are Lenvik (part of which is on the mainland), Berg, Torsken, and Tranøy.
The northern coasts of Senja face the open sea, the western coast faces the islands of Andøya and Krøttøya, and the southern coast faces the islands of Andørja and Dyrøya. On the western coast, steep and rugged mountains rise straight from the sea, with some fishing villages (like Gryllefjord and Husøy) tucked into the small lowland areas between the mountains and the sea.