river in Nord-du-Québec region, western Quebec province, Canada. It rises from Mistassini Lake in the central part of the province and flows generally westward for 380 miles (610 km) through a series of lakes to Ruperts Bay, at the south end of James Bay. It was discovered by Henry Hudson, who wintered at its mouth (1610–11), and was eventually named after Prince Rupert, first governor of the Hudson's Bay Company. The company's first trading post, Fort-Rupert (formerly Rupert House), was established at the river's mouth in 1668; other former posts along the river included Nemiscau and Mistassini Post.