| Rumoi River | |
| Name Other: | 留萌川 |
| Subdivision Type1: | Country |
| Subdivision Name1: | Japan |
| Subdivision Type2: | State |
| Subdivision Name2: | Hokkaidō |
| Subdivision Type3: | Region |
| Subdivision Name3: | Rumoi |
| Subdivision Type4: | District |
| Subdivision Name4: | Rumoi |
| Length: | 44km (27miles) |
| Discharge1 Avg: | 13.3m3/s |
| Source1: | Mount Poroshiri |
| Source1 Location: | Rumoi, Hokkaidō, Japan |
| Source1 Coordinates: | 43.9628°N 141.8361°W |
| Source1 Elevation: | 730.2m (2,395.7feet) |
| Mouth: | Sea of Japan |
| Mouth Location: | Rumoi, Hokkaidō, Japan |
| Mouth Coordinates: | 43.9561°N 141.6425°W |
| Mouth Elevation: | 0m (00feet) |
| Basin Size: | 270km2 |
is a river in Hokkaidō, Japan. The Rumoi is 44km (27miles) in length. It traces its source to Mount Poroshiri 731m (2,398feet) in the Hidaka Mountain range, and flows across Rumoi Subprefecture in the west of Hokkaidō and empties into the Sea of Japan. The mouth of the Rumoi River is in the city of Rumoi.[1]