The island is separated by a 16-kilometre (10 mi) stretch of sea from the nearest point of the mainland, the promontory of Monte Argentario. Mainly mountainous, it consists almost entirely of granite, culminating in the Poggio della Pagana, which rises to 496 metres (1,627 ft). Ninety percent of its surface is covered by Mediterranean vegetation, alternating with large pine forests and numerous vineyards which allow the production of the local "Ansonaco" wine. The coast is 27 kilometres (17 mi) long, made up of rocks, smooth cliffs and several bays: Arenella, Cannelle, Caldane and Campese, the biggest one with its small village of the same name.
The municipality is composed of the islands of Giglio and Giannutri. Three principal settlements are located on the main island:
- Giglio Porto (G. Harbour), located on the eastern coastal side and hosts the port. It is divided into the quarters of Chiesa, Moletto and Saraceno.
- Giglio Castello (G. Castle), located upon a hill between the two other localities and characterized by the majestic walls of a fortress. It is divided into the quarters of Casamatta, Centro, Cisterna and Rocca.
- Giglio Campese, it is located on the north-western coastal side and is a modern sea resort.