The Mrassu or Mras-Su ( or ÃÂÃÂðÃÂ-áÃÂÃÂ, ) is a river in Kemerovo Oblast, Russia. It is a left tributary of the Tom (in the Ob's drainage basin), and has its sources in the Abakan Range. It is long, with a drainage basin of . The Mrassu joins the Tom near Myski.
The Mrassu freezes from mid-November to April. The water flow at Myski averages at 155 square meters per second every year with a minimum of in February and a maximum of in May.