Cephissus is a river flowing through the Athens agglomeration, Greece.

The source of the river is in the saddle between the Parnitha and Penteli mountains. From there it flows generally southwest until it flows into the Phaleron Bay between Neo Faliro and Moschato. Today the river flows near or under the Motorway 1 linking Athens and Thessaloniki for much of its length. This section of Motorway 1 is named Kifissou Avenue.