The Tajuña (Spanish: [taˈxuɲa] ⓘ; from the Latin Tagonius) is a river in central Spain, flowing through the provinces of Guadalajara and Madrid.

It rises in the Sierra de Solorio, near the town of Maranchón (Guadalajara), at a location known as Fuente del Carro near the village of Clares.

It crosses the villages of Luzón, Anguita, Luzaga, Cortes de Tajuña, Brihuega and Renera in Guadalajara province and enters Madrid province at Ambite; it then passes close to Orusco, Carabaña, Tielmes, Perales de Tajuña and Morata de Tajuña, before joining the Jarama near Titulcia.

Being an internal river, its tributaries in the province of Guadalajara are seasonal streams which only contain water in spring.

The Tajuña gets its water from springs and from the Calizas del Páramo de la Alcarria aquifer which covers 1,800 km² at a depth of 30–40 m.