Zócalo (Latin American Spanish pronunciation: [ˈsokalo]) is the common name of the main square in central Mexico City. Prior to the colonial period, it was the main ceremonial center in the Aztec city of Tenochtitlan. The plaza used to be known simply as the Plaza Mayor ("Main Square") or Plaza de Armas ("Arms Square"), and today its formal name is Plaza de la Constitución ("Constitution Square").

This name does not come from any of the Mexican constitutions that have governed Mexico but from the Cádiz Constitution, which was signed in Spain in the year 1812. Even so, it is almost always called the Zócalo today. Plans were made to erect a column as a monument to independence, but only the base, or zócalo (meaning "plinth"), was built. The plinth was buried long ago, but the name has lived on. Many other Mexican towns and cities, such as Oaxaca, Mérida, and Guadalajara, have adopted the word zócalo to refer to their main plazas, but not all.

It has been a gathering place for Mexicans since Aztec times, having been the site of Mexican ceremonies, the swearing-in of viceroys, royal proclamations, military parades, Independence ceremonies, and modern religious events such as the festivals of Holy Week and Corpus Christi. It has received foreign heads of state and is the main venue for both national celebrations and national protests. The Zócalo and surrounding blocks have played a central role in the city's planning and geography for almost 700 years. The site is just one block southwest of the Templo Mayor, which, according to Aztec legend and mythology, was considered the center of the universe.

The modern Zócalo in Mexico City is 57,600 m² (240 m × 240 m). It is bordered by the Mexico City Metropolitan Cathedral to the north, the National Palace to the east, the Federal District buildings to the south and the Old Portal de Mercaderes to the west, the Nacional Monte de Piedad building at the northwest corner, with the Templo Mayor site to the northeast, just outside view. In the centre is a flagpole with an enormous Mexican flag ceremoniously raised and lowered each day and carried into the National Palace. There is an entrance to the Zócalo/Tenochtitlan metro station located at the northeast corner of the square, but no sign above ground indicates its presence.

Prior to the conquest, the area that the Zócalo occupies was open space, in the center of the Aztec capital Tenochtitlan. It was bordered to the east by Moctezuma II's "New Houses" or Palace (which would become the National Palace) and to the west by the "Old Houses", the palace of Axayacatl (1469–1481) where the Emperor Ahuitzotl, Moctezuma's uncle and immediate predecessor also lived. A European-style plaza was not part of the conquered Aztec Tenochtitlan; the old city had a sacred precinct or teocalli which was the absolute center of the city (and the universe, according to Aztec belief), but it was located to the immediate north and northeast of the modern-day Zócalo.

The current Zócalo occupies a space south-southwest of the intersection of roads that oriented Tenochtitlan. The north–south road was called Tepeyac–Iztapalapa, for the locations north and south it led to. The Tlacopan road led west and stretched east a little before leading into the lake that surrounded the city at the time. These roads were the width of three jousting lances according to Hernán Cortés. This intersection divided the city into four neighborhoods. The sacred precinct, containing the Templo Mayor, was located to the northeast of this intersection and walled off from the open area for commoners. As to this area's relationship to the teocalli proper, some historians say that it was part of it, but others say no.

The modern plaza of Mexico City was placed by Alonso Garcia Bravo shortly after the invasion when he laid out what is now the historic center. After the destruction of Tenochtitlan, Cortés had the city redesigned for symbolic purposes. He kept the four major neighborhoods or capullis but he had a church, now the Cathedral of Mexico City, built at the place the four adjoined. He had the Temo become the cathedral. The southern half was called the Plaza Mayor (Main Square) and the northern one was called the Plaza Chica (Small Square). Fairly early in the colonial period, the Plaza Chica would be swallowed up by the growing city.

During early colonial times, the plaza was bordered to the north by the new church, and to the east by Cortés's new palace, built over and with the ruins of Moctezuma's palace. On the west side of the plaza, the Portales de Mercaderes (Merchants' Portals) were built, south of Cortés' other palace, the Palace of the Marquis of the Valley of Oaxaca. On the south side, was the Portal of the Flowers (Flores), named so after its owner, María Gutiérrez Flores de Caballerías. Next to this portal was the House of the Ayuntamiento, a government building for the city. Both of these were behind a small drainage canal that ran east–west.