Mixco Viejo ("Old Mixco"), occasionally spelt Mixcu Viejo, is an archaeological site in the north east of the Chimaltenango department of Guatemala, some 50 kilometres (31 mi) to the north of Guatemala City and 4 kilometres (2.5 mi) from the junction of the rivers Pixcaya and Motagua. It is a moderate sized ruined city of the Postclassic Maya civilization.

The archaeological site and tourist attraction of Mixco Viejo was named after being erroneously associated with the Postclassic Poqomam capital referred to in colonial records by that name. The archaeological site has now been identified as Jilotepeque Viejo, the capital of the Chajoma Kaqchikel kingdom. To distinguish between the two, the ruins of the Chajoma capital are now referred to as Mixco Viejo (Jilotepeque Viejo) while the former Poqomam capital is referred to as Mixco Viejo (Chinautla Viejo).

This confusion in the identification of the site has hindered study. The Chajoma capital has been investigated archaeologically, under the assumption that it was the Poqomam capital. Although the Chajoma ruins of Jilotepeque Viejo have been well described archaeologically, the archaeological data has been associated with the history of a different site entirely. Doubts about the identification of the archaeological site were first raised by Robert M. Carmack, who realised that the supposed Poqomam capital was not located within the Poqomam linguistic area but rather within the linguistic area of the Kaqchikels. The Poqomam who were settled in the new colonial settlement of Mixco by the Spanish had a long history of fine polychrome ceramic production, but no evidence of such production had been recovered during archaeological investigations, and the ruins were considered too distant from colonial Mixco.

Mixco is a municipality in the Guatemala department of Guatemala.

Located next to the main Guatemala City municipality, over time it has become part of the Guatemala City Metropolitan Area. Most of Mixco is separated from the city by canyons, for which a multitude of bridges have been created. Ciudad San Cristóbal, one of Guatemala's largest cities, is located in this municipality. It is the third largest city in Guatemala, after Guatemala City and Villa Nueva, with a population of 688,124.

Territorially the municipality is divided into eleven areas, formed by colonies, villages, counties and the county seat. However, some villages are turned into colonies.

La Florida was once considered part of Mixco, but became part of Guatemala City in 1958.

Deportivo Mixco football club play in the Guatemalan second division. They have been playing their home games at different locations, since they plan to build the new Estadio de La Tierra de Campeones.