Tilos is situated between the islands of Nysiros and Halki, at a distance of 290 nautical miles from the port of Pereaus. It has an area of 64 square kms and a coastline of 63 kms. The ground is rocky with the only exception being a small fertile valley found almost at the centre of the island and which ends up on the idyllic, one kilometre long, sandy beach of Erystou.
At the north-west end of the island, the Monastery of Áyios Pandeleímon, (also the island's patron saint), sits on the slopes of Mount Profítis Ilías (654 m). The mountain borders a fertile plain running across the island's width, with the settlements of Áyios Andónis to the north and Éristos to the south. To the north-east of the plain is the island's capital, Megálo Chorió, built in the early 19th century at the foot of the ancient city of Telos. The archaic ruins stretch up to the site of the acropolis of the ancient city, dedicated to Pythios Apollo and Poliada Athina, and the Venetian Kástro, built over it. To the west is Kharkhadió Cave, where excavations in 1971 unearthed Neolithic finds and bones of dwarf elephant.
Above the cave stand the ruins of the medieval Fortress of Mesariá. At southern end of the island, bordered by more fertile meadows, is Livádhia, the major harbour and economic centre of the island. The island's old capital, Mikró Chorió, first settled in the 15th century by the Knights of the Order of St John, overlooks the bay. It has been completely abandoned since 1960, its inhabitants having moved down to the harbour in the 1930s. A number of other settlements such as Lethrá, Gherá, and Panó Méri have similarly been abandoned. Mount Áyios Nikoláos (367 m) stands to the south of the bay.