| Länggrien | |
| Location: | River Aare |
| Map: | Switzerland |
| Elevation M: | 428 |
| Country: | Switzerland |
| Country Admin Divisions Title: | Canton |
| Country Admin Divisions: | Solothurn |
| Country Admin Divisions Title 1: | District |
| Country Admin Divisions 1: | Lebern |
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.