Flat Bastion is a bastion which projects southward from the Charles V Wall in the British Overseas Territory of Gibraltar. Once known as the St. Jago's Bastion or the Baluarte de Santiago in Spanish, the fortification was built by the Spanish in the mid 16th century and formed part of the southern defences of the city of Gibraltar, together with Charles V Wall, Southport Gates, , and South Bastion. In 1859, six guns, four 12-pounders and two 12-pound carronades, were installed on the bastion, and four years later, five 32-pounders were mounted on the fortification.