Alcalá de Henares is a Spanish city in the Community of Madrid. Straddling the Henares River, it is located 35 kilometres (22 miles) to the northeast of the centre of Madrid. As of 2018, it has a population of 193,751, making it the region's third-most populated municipality.

Predated by earlier settlements (oppida) on the left bank of the Henares, the city has its origins in the Complutum settlement founded in Roman times on the right bank (north) of the river, that became a bishopric seat in the 5th century. One of the several Muslim citadels in the Middle Mark of al-Andalus (hence the name Alcalá, a derivative of the Arabic term for citadel) was established on the left bank, while, after the Christian conquest culminated circa 1118, the bulk of the urban nucleus returned to the right bank. For much of the late middle-ages and the early modern period before becoming part of the province of Madrid, Alcalá de Henares was a seigneurial estate of the archbishop of Toledo.

Its historical centre is one of UNESCO's World Heritage Sites.

The city has a long university tradition. Francisco Jiménez de Cisneros founded the Complutense University in Alcalá de Henares in the late 15th century. The city currently hosts the (refounded) University of Alcalá. It is the native city of Miguel de Cervantes.