Teotihuacan also written Teotihuacán , About this sound modern Nahuatl pronunciation (help·info)), is an ancient Mesoamerican city located in a sub-valley of the Valley of Mexico, located in the State of Mexico 40 kilometres (25 mi) northeast of modern-day Mexico City, known today as the site of many of the most architecturally significant Mesoamerican pyramids built in the pre-Columbian Americas.

At its zenith, perhaps in the first half of the 1st millennium AD, Teotihuacan was the largest city in the pre-Columbian Americas, with a population estimated at 125,000 or more, making it at least the sixth largest city in the world during its epoch.

Apart from the pyramids, Teotihuacan is also anthropologically significant for its complex, multi-family residential compounds, the Avenue of the Dead and the small portion of its vibrant murals that have been exceptionally well-preserved. Additionally, Teotihuacan exported fine obsidian tools that garnered high prestige and widespread usage throughout Mesoamerica.

The city is thought to have been established around 100 BC, with major monuments continuously under construction until about 250 AD. The city may have lasted until sometime between the 7th and 8th centuries AD, but its major monuments were sacked and systematically burned around 550 AD.

Teotihuacan began as a new religious centre in the Mexican Highlands around the first century AD. This city came to be the largest and most populated centre in the pre-Columbian Americas. Teotihuacan was even home to multi-floor apartment compounds built to accommodate this large population. The term Teotihuacan (or Teotihuacano) is also used for the whole civilization and cultural complex associated with the site.

Although it is a subject of debate whether Teotihuacan was the centre of a state empire, its influence throughout Mesoamerica is well documented; evidence of Teotihuacano presence can be seen at numerous sites in Veracruz and the Maya region. The later Aztecs saw these magnificent ruins and claimed a common ancestry with the Teotihuacanos, modifying and adopting aspects of their culture. The ethnicity of the inhabitants of Teotihuacan is also a subject of debate. Possible candidates are the Nahua, Otomi or Totonac ethnic groups. Scholars have also suggested that Teotihuacan was a multi-ethnic state.

The city and the archaeological site are located in what is now the San Juan Teotihuacán municipality in the State of México, approximately 40 kilometres (25 mi) northeast of Mexico City. The site covers a total surface area of 83 square kilometres (32 sq mi) and was designated a UNESCO World Heritage Site in 1987. It is the most visited archaeological site in Mexico.