Alcalá de Henares is a Spanish city. Located in the autonomous community of Madrid, 30 km northeast of the city of Madrid, it has a population of around 200,000.

The city is of Roman origin - the only Roman town in the Madrid region - its Latin name being Complutum. It was refounded in 1083 by the Moors, who built a castle or al-qalat on a nearby hill, today known as Alcalá la Vieja (Old Alcalá). Its Christian conquerors preferred the Burgo de Santiuste ("Saint Just's borough") on the original Roman site. The city was ceded to the Bishopric of Toledo, Spain. The present name literally means "castle on the [river] Henares". Under Christian rule, the city sported both a Jewish and a Moorish quarter.

At some time in the 1480s Christopher Columbus had his first meeting here with the Reyes Católicos, Ferdinand and Isabella. In 1496, Cardinal Cisneros founded the Universidad Complutense, which became famous as a centre of learning during the Renaissance. It was moved to Madrid in 1836 (under the name Universidad Central de Madrid). A new university was founded in the old buildings as the Universidad de Alcalá de Henares in 1977. The city suffered severe damage during the Spanish Civil War.

The author Miguel de Cervantes was born in Alcalá de Henares, as were Ferdinand I of Aragon and Catherine of Aragon, the first wife of Henry VIII of England. Other notable figures associated with the city include the theologian Gabriel Vázquez, the mystic John of the Cross, the Holy Roman Emperor Ferdinand I, and President Manuel Manuel Azaña, the last president of the Second Spanish Republic.

Alcalá's excellent transport links with Madrid have led to its becoming a commuter town, with many of its inhabitants travelling to work in the capital. It was affected particularly badly by the March 11, 2004 terrorist attacks in Madrid as the bombed trains all originated at or passed through Alcalá.

A large part of the new population are immigrants from Eastern Europe.

Alcalá de Henares, as the birthplace of Catherine of Aragon, is twinned with the city of Peterborough in the United Kingdom, her final resting-place.