Following Barcelona's protracted resistance to the siege by Franco-Spanish troops, which was finally crushed on September 11th, 1714, the conquering Philip V felt the need to stamp his authority on Barcelona and thereby subject the whole of Catalonia. Today's Parliament Palace is the former arsenal of the fort he had built in order to achieve that aim.

The building work, which started on March 1st, 1716 and lasted until 1748, was directed by the Flemish military engineer Próspero de Verboom (1665-1744), also responsible for the fort of Antwerp, and one of the most distinguished officers in the armies of Louis XIV and Philip V. Later, Verboom became the first governor of the Citadel (Ciutadella), a position he held until his death.

The fort, of pentagonal design, was not for use in the defence of the city of Barcelona against an army. Between 1719 and 1866, this Catalan 'Bastille' was where scores of political prisoners were executed, on the esplanade and the earthworks of the Citadel.

The huge public outcry against the fort in the revolutionary atmosphere of the nineteenth century was answered with a law that ceded the building to the city, and its demolition was ordered in 1869. Only the arsenal still stands, along with the governor's palace (today the Verdaguer Institute) and the chapel (now a military parish church).

The arsenal building essentially conserves the structure and appearance given it by Próspero de Verboom. It measures 5,532 square metres and has two floors and a loft. The interior consists of two robust naves in the form of a cross, with a dome rising above the central pavilion. Between the arms of the cross are four courtyards. On the outside, the front of the central body and the wings are extended by a series of arcades that form a portico around the first floor. The materials used throughout the building are stone from Montjuïc and red brick.

In 1889, Barcelona City Council agreed to convert the former arsenal of the Citadel into a royal palace. The adaptation work started on September 28th under the direction of Pere Falqués (1850-1916), the city architect of Barcelona. This entailed opening three balconies at first floor height, decorating the whole facade with sgraffiti, building the central body of the Palace, and repositioning the stone crest of the Socors Gate into the Citadel in the middle of the facade. Pere Falqués also designed the famous street lamps in Passeig de Gràcia, which together with his other work has assured him a place amongst his contemporary Art Nouveau Catalan architects, Antoni Gaudí and Lluís Domènech i Montaner.

The Queen Regent having declined to accept the transfer of the building, in 1900 the Municipal Commission of Government proposed using it as the city art gallery. The building soon had to be extended: two wings were added, also made out of stone from Montjuïc and red brick. The facades were decorated with twenty-four busts of Catalan artists and figures related to the history of Catalan art. This extension was opened in 1915. When the park was restored in 1927, the current garden was created, with a lake in the centre containing a replica of the sculpture Desconsol (Distress), by Josep Llimona.

On October 14th, 1932, the City Council ceded the palace so that it could become the seat of the Catalan Parliament.Santiago Marco directed the work of adapting and decoration, co-ordinated by Josep Tarradellas, the Government Minister. On December 6th, 1932, the solemn inaugural sitting was held, in the presence of the President of the Generalitat, Francesc Macià.

On January 26th, 1939, on the entry of General Franco's troops into Barcelona, the Parliament Palace was turned into a barracks. Later, in 1945 the Museum of Modern Art was established there, and later still, the Numismatic Office and the Art Museums Library. As a symbol of the punishment imposed on the whole nation, the Saló de Sessions was closed, and for thirty-seven years its use was strictly restricted.

In April 1980 under the co-ordination of Government Minister Josep M. Bricall, the Palace was restored and adapted to be ready for the start of the first legislature of the newly re-established Catalan Parliament.

At the end of the 5th legislature, Jordi Pujol, President of the Generalitat, and Joan Reventós, President of Parliament, signed an agreement with Joan Clos, Mayor of Barcelona, under which the Palace building in the park was donated for use by the Generalitat for as long as it remained the seat of Parliament.

This signalled the start of the great transformation that took place under the presidency of Joan Rigol during the 6th legislature, continued and furthered since 2003 by Ernest Benach, the current President of Parliament. There has been a gradual extension of available space by the recovery of the rooms that until September 2004 were occupied by the Museum of Modern Art, after which the whole Palace was destined exclusively to parliamentary purposes.

On July 20th, 2006, Pasqual Maragall, President of the Generalitat, Ernest Benach, President of Parliament, and Joan Clos, Mayor of Barcelona, signed a framework agreement for collaboration to extend the Parliament's premises with a new building. This was contemplated in the future modification of the general metropolitan plan for the whole of Ciutadella Park, and met the specifications and criteria defined in the park's master plan.