Sompasaari (Swedish: Sumparn) is an island on the Kruunuvuorenselkä water area in the Sörnäinen district in Helsinki, Finland. On the southern side of the Nihdinkanava channel built in the middle of Sompasaari is the island of Nihti.
The island was originally connected to the mainland with reclaimed land and the Sörnäinen Harbour was built on top of it. Because of this, the entire harbour has been referred to as Sompasaari. Signs on roads leading to the harbour used the name Sompasaari exclusively, with an icon representing a cargo harbour. Harbour facilities in Sörnäinen ended in late 2008 when they moved to the new Vuosaari Harbour. In 2016 the island was converted to a residential area and it was separated from nearby Kalasatama by digging a new channel called Sompasaarenkanava between them.