It is a Gothic building dating from 1302, located in the Plaça del Rei in the centre of Barcelona's Gothic Quarter. Today, the Salón del Tinell and the Chapel of Santa Ágata form part of the Barcelona History Museum.

It was ordered to be built by King James II of Aragon and his wife Blanca of Naples to form part of the Palau Reial Major to replace the old oratory in the palace.

The master builder was Bertran Riquer, followed in 1316 by Jaume del Rei and later by Pere d'Olivera. Construction continued under the reign of Peter the Ceremonious, who ordered the construction of the Queens' Chapel in the transept of the church; under King Martin I the Humane, the chapel that served as a baptistery was built at the foot of the nave. Its original dedication was in honour of Saint Mary, until the year 1601, when a papal bull was issued to change its name due to the relics of Saint Agatha being kept inside.

The chapel suffered the disentailment of 1835 and was used for various purposes. In 1856, a restoration was begun by the architect Elías Rogent. From 1879 onwards, the Provincial Museum of Antiquities was set up in the chapel, with a large lapidary collection. In the 1990s, a new restoration was carried out by the architects Jordi Casadevall and Alfred Pastor, with work on the roof, the ceiling, the lighting and finally the consolidation of the bell tower.

Its architecture is in the Catalan Gothic style dating from 1302, built on top of a Roman wall. It consists of a single rectangular nave with a polygonal apse, a small transept with the chapel of the Queens, where the coats of arms of Maria of Navarre and Leonor of Sicily, wives of King Peter the Ceremonious, can be seen. At the foot of the nave is a small chapel that served as a baptistery. The nave has a double-sloped roof with a polychrome wooden coffered ceiling. The apse is covered with ribbed vaults and behind it is the sacristy excavated inside the Roman wall. Located between the buttresses are stained-glass windows with Gothic tracery that illuminate the interior space. Above the part of the sacristy is the octagonal bell tower, with its eight triangular pediments terminating in the shape of a royal crown, which was built in the first quarter of the 14th century.

As a curiosity, the sculptor Juan Claperós, under the mandate of King Pedro of Portugal (1463-1466), made sixty terracotta tiles with representations of angels and the arms of Aragon and Sicily for the paving, and this same king commissioned the magnificent altarpiece of the Epiphany, which presides over the chapel, from the artist Jaume Huguet.