Antillia or Antilia is a phantom island that was reputed, during the 15th-century age of exploration, to lie in the Atlantic Ocean, far to the west of Portugal and Spain. The island also went by the name of Isle of Seven Cities (Portuguese: Ilha das Sete Cidades; Spanish: Isla de las Siete Ciudades).

It originates from an old Iberian legend, set during the Muslim conquest of Hispania c. 714. Seeking to flee from the Muslim conquerors, seven Christian Visigothic bishops embarked with their flocks on ships and set sail westwards into the Atlantic Ocean, eventually landing on an island (Antillia) where they founded seven settlements.

The island makes its first explicit appearance as a large rectangular island in the 1424 portolan chart of Zuane Pizzigano. Thereafter, it routinely appeared in most nautical charts of the 15th century. After 1492, when the north Atlantic Ocean began to be routinely sailed and became more accurately mapped, depictions of Antillia gradually disappeared. It nonetheless lent its name to the Spanish Antilles.

The routine appearance of such a large "Antillia" in 15th-century nautical charts has led to speculation that it might represent the American landmass, and has fueled many theories of pre-Columbian trans-oceanic contact.

Stories of islands in the Atlantic Ocean, legendary and otherwise, have been reported since classical antiquity. Utopian tales of the Fortunate Islands (or Isles of the Blessed) were sung by poets like Homer and Horace. Plato articulated the utopian legend of Atlantis. Ancient writers like Plutarch, Strabo, and, more explicitly, Pliny the Elder and Ptolemy, testified to the existence of the Canary Islands. The names of some real islands re-emerged as distinct mythical islands with associated legends, e.g. capraria (the island of goats) and canaria (the island of dogs) are often found on maps separately from the Canary Islands (e.g. Pizzigani brothers, 1367).

The Middle Ages saw the emergence of Christian versions of these tales. Notable among these are the Irish immrama (tales of a hero's journey to the Otherworld), such as the immram of Uí Corra, or the sea voyages of the 6th-century Irish missionaries Saint Brendan and Saint Malo. These are the source for several legendary Atlantic islands such as Saint Brendan's Island and the Island of Ima. The sagas of Norse seafarers to Greenland and Vinland, notably the Grœnlendinga saga and the saga of Erik the Red, have also been influential. Norse encounters with North American indigenous peoples seem to have filtered into Irish immrama.

The peoples of the Iberian Peninsula, who were closest to the real Atlantic islands of the Canaries, Madeira and Azores, and whose seafarers and fishermen may have seen and even visited them, articulated their own tales. Medieval Andalusian Arabs related stories of Atlantic island encounters in the legend (told by al-Masudi) of the 9th-century navigator Khashkhash of Cordoba and the 12th-century story (told by al-Idrisi) of the eight Maghrurin (Wanderers) of Lisbon.

Given the tendency of the legends of different seafarers – Greek, Norse, Irish, Arab and Iberian – to cross-fertilize and influence each other, the exact source of some legendary Atlantic islands – such as the mythical islands of Brasil and the Isle of Mam – are extremely difficult to disentangle.