Sermitsiaq Island (Danish: Sadelø) is an uninhabited island in the Sermersooq municipality in southwestern Greenland.

Sermitsiaq is one of three mountainous islands located in the middle section[1] of the 160 km (99.4 mi) long[2] Nuup Kangerlua fjord, to the north of Nuuk, the capital of Greenland.

The two sibling islands are Qeqertarsuaq Island and Qoornuup Qeqertarsua Island.

[1] The prominent 1,210 metres (3,970 ft)[2] Sermitsiaq mountain topping the island is a landmark of Nuuk, visible from most places in the city.