The municipality is located in the Agro Pontino, about 100 km south of Rome and its territory has an extension of 3,139 hectares (it ceded to Sabaudia 690 hectares of woodland upon its foundation (1934) - in exchange it was assigned 640 hectares of the territory of Terracina).
The municipal territory is spread over two well-defined areas: the Circeo promontory, 6 km, and the flat part.
The promontory appears from Anzio as the profile of a face of a reclining woman (identified with the sorceress Circe, in which the peak "Picco di Circe" corresponds to her nose), while from Gaeta it appears as an island, the ancient Isola Eea. The following peaks rise along the ridge of the promontory: Vetta di Paola, Picco di Circe (541 m, the maximum height above sea level), Punta di Torre Moresca, Punta di Vasca Moresca, Punta del Fortino di Cretarossa, Punta del Semaforo (412 m), Punta dell'Acropoli or delle Crocette (352 m). There are also various hills that rise on the eastern side and surround the historic center: Peretto, Guardia Orlando and Monticchio to the north, Monte Morrone and S. Rocco to the south. Various geological investigations have shown that the promontory is mostly made up of limestones of various types (crystalline and white, compact, flint with brachiopods, clayey).
The plain below the promontory instead consists of submerged sands, reclaimed during the Fascist period. In the Quaternary the promontory was truly an island and there are various proofs of this: the discovery on the promontory of a gray sandstone which was later found only at a depth of 300 metres, the marine fossil furrows, the signs of marine erosion and the information obtained from study of the Pliocene strata.