The Islote de Sancti Petri is a small barren island belonging to the municipality of San Fernando, Cádiz in the province of Cádiz and the autonomous region of Andalusia, Spain. It is close to the coast and 16 kilometres (10 mi) south of Cadiz city.

For many years, the island was a restricted military zone, but it is now under the control of the Spanish Environment Ministry. In the first century BC, the Greek historian Strabo mentioned in his book Geographica that the Tyrians founded Gadeira, building a sanctuary to Melkart, known as Hercules to the Greeks, in the eastern part of the island and the city in the western part.

Castle

Sancti-petri castle
Native name Spanish: Sancti-petri
Location of Sancti-petri castle in Province of Cádiz Show map of Province of Cádiz Islote de Sancti Petri (Andalusia) Show map of Andalusia Islote de Sancti Petri (Spain) Show map of Spain
Location	San Fernando, Cádiz, Spain
Coordinates	36°22′47″N 6°13′13″W / 36.379607°N 6.220177°W / 36.379607; -6.220177
Spanish Cultural Heritage
Official name	Sancti-petri castle
Type	Non-movable
Criteria	Monument
Designated	1993
Reference no.	RI-51-0007584

The ruins of an eighteenth-century castle remain on the island; the tower of the castle has been substantially rebuilt, and it is now in use as a lighthouse. It was declared Bien de Interés Cultural in 1993.^{[citation needed]}

The name, Sancti Petri, also applies to the beach on the mainland, in Chiclana de la Frontera, directly opposite the island.