Skiathos is the westmost island of Sporades with a distance of 41 nautical miles from Volos and just 2,4 nautical miles from the coastline of mainland (Pillion). In terms of public administration Sporades Group is part of Magnisia Prefecture where Volos is the capital. All previous are part of Thessalia Area.

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. Other main settlements are Kalyvia (312), Troullos (158), Χanemos (143), Koukounaries (119), and Achladias (118). 

The Municipality of Skiathos includes the islets of Tsougria, Tsougriaki, Maragos, Arkos, Troulonisi and Aspronisi. The municipality has an area of 49.898 km2.  They are scattered a few kilometres off the southeast coastline and are clearly visible from the town and the beaches. The island of Skopelos is visible from Skiathos with the more distant islands of Euboea and Skyros visible under very clear weather conditions.

Skiathos town

The main paved road runs along the southeastern stretch of the island with several narrow dirt roads branching off towards the interior and the northern coast. Farmland exists around the major settlements on the island.

Despite its small size, Skiathos with its many beaches and wooded landscape is a popular tourist destination. It has over 60, mostly sandy, beaches scattered around the 44 km (27 mi) coastline. Some of these are Troulos, Vromolimnos, Koukounaries, Asselinos, Megali Ammos and Mandraki. 

Much of the island is wooded with Aleppo Pine and a small Stone Pine forest at the Koukounaries location where there is a lagoon and a popular beach. The island's forests are concentrated on the southwest and northern parts, but the presence of pine trees is prevalent throughout the island.