The Ruins of Fort St. Barbara (Spanish: Ruínas del Fuerte de Santa Bárbara) are the ruins of a fort located in La Línea de la Concepción, Spain.

This fortification was constructed by the Spanish after Gibraltar was formally ceded to the United Kingdom after being captured by an Anglo-Dutch force in 1704.

The agreement to cede the isthmus was part of the Treaty of Utrecht and Spain gave Britain Menorca, Gibraltar and the right to sell slaves to Spanish colonies.

As a result, a defensive line (hence La Línea) was built at the north end of the isthmus joining the Rock of Gibraltar to Spain in order to keep the British from attacking or encroaching on Spain.

The main Spanish lines were destroyed by Colonel Sir Charles Holloway on 14 February 1810.