The Mrassu or Mras-Su (Russian: Мрассу́ or Мрас-Су́, Shor: Прас, Пырас, Пырас-суғ, romanized: Pras, Pıras, Pıras-suq) 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 338 kilometres (210 mi) long, with a drainage basin of 8,840 square kilometres (3,410 sq mi). 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 (1668 ft/sec) every year with a minimum of 24.4 m/sec (262 ft/sec) in February and a maximum of 694 m/sec (7470 ft/sec) in May.