Guadalix de la Sierra is a municipality of the Community of Madrid, Spain, on the Jarama river around 49 kilometres from the city centre.

In October 2024, archaeologists working on an excavation from the Autonomous University of Madrid found an ancient Visigoth funeral basilica with seventy tombs inside and surrounding the building, dating from around the 7th century CE.

The town stayed rural throughout, however the construction of a reservoir in El Vellón in 1967 flooded traditional livestock trails and pasture zones.

The municipality now has a more urban outlook while maintaining remnants of the rural style it traditionally had.

Since 2001, the area has been known as the location of the Gran Hermano house, so much so that Guadalix is often used as a metonym for the series.