James Bay, shallow southern arm of Hudson Bay, c.300 mi (480 km) long and 140 mi (230 km) wide, E central Canada, in Nunavut Territory between Ont. and Que. Numerous rivers flow into the bay; many of these have been developed for hydroelectric power in Quebec (see James Bay Project). Of its many islands, the largest is Akimiski (1,158 sq mi/3,000 sq km). The shores of the bay and some of its islands are wildlife reserves.
The bay was discovered (1610) by Henry Hudson but was named for Capt. Thomas James, an Englishman who explored much of it in 1631. An early fur-trading post established by Groseilliers and Radisson became (1670) Rupert House, the first post established there by the Hudson's Bay Company. Other important posts on James Bay were Fort Albany, Fort George (now Chisasibi), and Eastmain.