The Alcantarilla Dam is a ruined Roman gravity dam in Mazarambroz, Toledo province, Castilla-La Mancha, Spain, dating to the 2nd century BC. The toponym "Alcantarilla" means conduit and is of Arabic origin: the Latin name is unknown.The dam is believed to be the oldest dam in Spain, and possible the oldest Roman dam. It was 20 metres (66 ft) high and at least 550 metres (1,800 ft) long. The dam and reservoir were part of the water supply system of the city of Toletum (Toledo).
The structure appears to have been similar to the surviving Proserpina Dam near Merida, an earth dam with a stone retaining wall. The upstream retaining wall consists of two parallel rubble-masonry walls about 1 metre (3.3 ft) thick, separated by a concrete-filled space approximately .6 metres (2.0 ft) wide. The upstream side of the wall was faced with cut stone blocks.
The structure has been in ruins for a long time. There has been speculation that it was not strong enough to cope with a large volume of water. It was possibly breached in the Roman era. Another possibility is that the masonry collapsed upstream, perhaps by the pressure of the earth fill when the water was low, since, unlike later dams, it was not buttressed on the upstream side.The remains of Toledo's Roman water supply system are partly protected by a heritage designation (Bien de Interés Cultural).