Qassiarsuk is a settlement in the Kujalleq municipality, in southern Greenland. Its population was 89 in 2010.
Brattahlíð is located by Tunulliarfik Fjord (Skovfjorden in Danish), and it was the site of Erik the Red's estate in the times of the Norse Eastern Settlement (Austerbygda in Danish). Ruins of several of the buildings, including living quarters, outhouses, and a church, are still clearly visible.
The current village was founded as Greenland's first sheep farm in 1924 and is located in the same place.
Until December 31, 2008, the settlement was part of Narsaq Municipality in the Kitaa amt. On January 1, 2009, Qassiarsuk became part of Kujalleq municipality, when the Kitaa amt, as well as the municipalities of Narsaq, Qaqortoq, and Nanortalik ceased to exist as administrative entities.