The Plaza de la Constitución is a square in the Old Quarter of San Sebastian. The city's major festivals and events, such as the tamborrada of San Sebastián or the fair on the day of Santo Tomás, are usually held in this square, which has arcades as its main feature. Its central building was the city hall until 1947, when it was occupied by the Municipal Library (1951-2000) and, later, by the administrative offices of the City Council's cultural department.

Throughout its history, the square has been known as Plaza Nueva, Plaza de la Constitución, Plaza del 18 de Julio, and Plaza de la Constitución again.

There was already a square designed by Hércules Torrelli in 1723 on the site of the current Plaza de la Constitución. After the city was burned down during the siege of San Sebastián in 1813, the new neoclassical square was built. Its dimensions are modulated in 20 equal arches on each of the larger sides and 9 arches on the smaller side, each equal to the first.

The square's balconies are numbered because they served as boxes for the bullfighting festivities held there.