The Cephissus or Kifisos (Greek: Κηφισός) is a river in central Greece. In Greek mythology the river god Cephissus was associated with this river. The river rises at Lilaia in Phocis, on the northwestern slope of Mount Parnassus. It flows east through the Boeotian plain, passing the towns Amfikleia, Kato Tithorea and Orchomenos. It drained into Lake Copais, which was therefore also called the Cephisian Lake, until 1887, when the lake was drained. An artificial outflow has been created to Lake Yliki (ancient Hylice), further east.
Pausanias records a Theban tradition that the river Cephissus formerly flowed under a mountain and entered the sea until Heracles blocked the passage and diverted the water into the Orchomenian plain; but he does not believe it.
Pausanias also says that the Lilaeans on certain days threw cakes and other customary items into the spring of the Cephissus and that they would reappear in the spring of Castalia.