The islands of São Tomé and Príncipe, situated in the equatorial Atlantic about 300 and 250 kilometres (186 and 155 mi), respectively, off the northwest coast of Gabon, constitute Africa's second smallest country. Both are part of the Cameroon volcanic mountain line, which also includes the islands of Annobón to the southwest, Bioko to the northeast (both part of Equatorial Guinea), and Mount Cameroon on the African west coast.

São Tomé is 50 km (31 mi) long and 32 km (20 mi) wide and the more mountainous of the two islands. Its peaks reach 2,024 metres (6,640 ft). Príncipe is about 30 km (19 mi) long and 6 km (4 mi) wide. Swift streams radiating down the mountains through lush forest and cropland to the sea cross both islands.

The equator lies immediately south of São Tomé Island, passing through an islet named Ilhéu das Rolas.