Dão is a Portuguese river that originates in the plateau regions of Trancoso-Aguiar da Beira in the Eirado parish, Aguiar da Beira Municipality, Guarda District, and varies between 714 and 757 metres in elevation.
It flows in a general northeast–southwest direction, passing through Barragem de Fagilde and crosses or demarcates the limits of the councils of Aguiar da Beira, Penalva do Castelo, Mangualde, Viseu, Carregal do Sal, Nelas, Tondela, and Santa Comba Dão. It empties into the Mondego River, in the limits of the councils of Santa Comba Dão, Mortágua, and Penacova, after covering about 100 kilometres.
