Antillia (or Antilia) was a phantom island said to lie in the Atlantic Ocean far to the west of Spain. This mythical island had several other names such as Isle of Seven Cities, Septe Cidades, Sanbrandan (or St Brendan), etc. Antillia was also identified with islands including the Isles of the Blest and the Fortunate Islands.

A Portuguese legend tells how the island was settled by the Archbishop of Porto accompanied by six bishops and their parishioners in either 714 or 734 in the face of the Moorish conquest of Iberia. The archbishop and bishops each founded a city, known as Aira, Anhuib, Ansalli, Ansesseli, Ansodi, Ansolli and Con. A similar Spanish tradition claims that these bishops were all Spanish. The Irish also have the very same tradition, which they ascribe to St Brendan, a real Irish saint to whom many such mythical feats are attributed.

The island is first known to have appeared on a map in 1424. It was later claimed that it had been sighted by a Spanish ship in 1414, while a Portuguese crew claimed to have landed on Antillia in the 1430s. Many expeditions were launched in an attempt to find the island, and in 1492 Christopher Columbus planned to stop there on his journey to Asia.

On maps, Antillia was typically shown as being almost the size of Portugal, lying around two hundred miles west of the Azores. It was an almost perfect rectangle, its long axis running north-south, but with seven or eight trefoil bays shared between the east and west coasts. This has made some scholars to identify the island as Puerto Rico. Each city lay on a bay. The similar island of Saluaga was shown north of Antillia, while Taumar and Ymana (or Roillo) lay nearby.

Some, the first being Peter Martyr d'Anghiera in 1493, believe that Antillia represented a previous discovery of the West Indies, and as a result the Caribbean islands became known as the Antilles. A less popular theory identifies the island with Sao Miguel in the Azores, where seven villages around twin lakes are known as Sete Cidades. Either way, this and the improving knowledge of the Atlantic led to Antillia shrinking on maps and disappearing entirely after 1587.