Onchestus or Onchestos (), was a river of ancient Thessaly, flowing near Scotussa, through the battlefield of Cynoscephalae into Lake Boebeis. It is perhaps the same river as the Onochonus or Onochonos (á½ÂýÃÂÃÂÃÂýÿÃÂ), whose waters were exhausted by the army of Xerxes I during the Persian invasion of Greece. It is true that Herodotus describes this river as flowing into the Peneius; but in this he was probably mistaken, as its course must have been into Lake Boebeis.