Länggrien (also known as Aareinseli) is an island in the River Aare, located in the canton of Solothurn, Switzerland. The island has a maximum length of 600 metres and a maximum width of 270 metres. Its highest point is 428 metres above sea level, a few metres above the river level. The minimum distance from the shore is about 50 metres.
Politically the island belongs to the municipality of Selzach in the district of Lebern.
