| Mrassu | |
| Image Alt: | A photograph of the Mrassu |
| Mouth: | Tom |
| Mouth Coordinates: | 53.7486°N 87.7986°W |
| Subdivision Type1: | Country |
| Subdivision Name1: | Russia |
| Length: | 663km (412miles) |
| Basin Size: | 17600km2 |
The Mrassu or Mras-Su (ru|Мрассу́[1] or Мрас-Су́, cjs|Прас, Пырас, Пырас-суғ|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 338km (210miles) long, with a drainage basin of 8840km2. 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.[2]