Somerset Island lies north of Boothia Penninsula and is separated from it by the narrow Bellot Strait. The island is divided into two topographic regions: a plateau of sedimentary rocks northeast of a line running from Hazard Inlet to Aston Bay; and, to the southwest, a rocky upland of Precambrian granite. North of Stanwell Fletcher Lake, a lowland area of Precambrian granite separates the two regions.
The limestone and sandstone bedrock forms a flat plateau in the northeast that occupies much of the island. Though it is mainly level, this plateau's elevation ranges between 300 and 600 m, and is cut by many deep, but narrow, river valleys. These rivers drain into the many bays that surround the plateau. In some places, erosion has caused hill-like structures to form; they are not true hills, however, as the tops are even with the rest of the plateau. The plateau's coastline is rugged: steep cliffs between Cunningham Inlet and Garnier Bay fall sharply into Barrow Strait.
The southwestern region of Somerset Island is an upland area, with a very hilly section south of Fitz Roy Inlet. North of the inlet, the terrain is still hilly, but dips gradually as it progresses north to Aston Bay. There are several small lakes interspersed among the hills.
The beginning of the lowlands is marked by Stanwell Fletcher Lake, which is flanked by escarpments, one from the highlands to its west and the other from the plateau to its east.
The only permanent settlement on Somerset Island is Fort Ross.