A beautiful triangular-shaped park located in the center of the city, this is one of the places where locals get together. On Thursdays and Sundays at 7:00 p.m., the plaza comes alive with the music of the State Band.

In colonial times, this plaza was called Plaza de San Diego because of its proximity to the convent and church of the Dieguinos. The plaza served as an outdoor market and for all the traditional festivities. At times it was used as a bullfighting ring.

In 1810, the meeting between the Dieguino Belaunzarán and the Count of Flón took place here. The first demanded in the name of God that the order to behead the Guanajuatan citizens not be carried out.

In 1836, the first of the plazas trees were planted: a row of ash trees. Since the city lacked gardens and walking paths, in 1861 the main garden was planted and paths made all around it with benches and oil street lamps. It officially received the name of Jardín de la Unión, by which it is known to this day. It was and continues to be the favorite place for locals to meet in the city. It was also the first place in the city to receive electricity. The garden's gazebo was built during the presidency of Porfirio Díaz. The Jardín is triangular in shape and quite small, making it easy to find and make friends.