The island has a north to southwestern axis and is about 12 kilometres (7 miles) long and 6 kilometres (4 miles) wide on average. The coastline is indented with inlets, capes and peninsulas. The southeast and southwest parts have gentler slopes and that is where most settlements and facilities are located. The terrain is more rugged on the north coast, with the highest peak at 433 m (1,421 feet) on mount Karafiltzanaka (39.1904°N 23.4685°E).
The main town is Skiathos (pop. 4,883 in 2011), in the eastern part of the island.