Cephissus (Ancient Greek: Κήφισσος; Greek: Κηφισός, Kifisos) is a river in the vicinity of Athens, Greece. Together with the neighbouring river Ilisos, it drains a catchment area of 420 km² (160 sq mi).^[1]

The Bibliotheca (3.15.1) states that the legendary Erechtheus' wife Praxithea was the daughter of Phrasimus (otherwise unknown to us) by Diogenia (otherwise unknown to us) daughter of Cephissus.

The source of the river is in the saddle depression between the Parnitha and Penteli mountains. From there it flows generally southwest until it reaches the Phaleron Bay between Neo Faliro and Moschato. Presently 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 Avenue Kifissou, near which is the Kifissos Bus Terminal.