The church of San Martín is located in Santa María de Buil, a town in the municipality of Aínsa-Sobrarbe, in the province of Huesca. The church is associated with the castle of Buil, which has been documented since 1017, and is located on the side of a hill.

The church, dedicated to San Martín de Tours, has the old abbey house (from the 18th century) attached to its south-west corner. The oldest parts of the building, such as the nave and tower, are thought to have been built between 1040 and 1050, and the apses are thought to have been built around 1070. During the 16th and 17th centuries modifications were made to the interior, such as the opening of a north chapel and the construction of a south arcade, and in the 18th century the interior was decorated with paintings. The church was restored in the 1970s.

The church has a basilica plan with three naves and three apses, as well as a tower at the foot. The naves are covered with barrel vaults, and the apses are covered with oven vaults. The interior of the church has extensive pictorial decoration. From the outside, the most outstanding elements are the apses and the tower, both with a characteristic design. The apses are set back from the nave and have a high base with pilasters and an irregular arched ambulatory. The tower, which originally had two sections, is related to Lombard architecture and has a third upper section added in the 16th century.