James Bay is the southern appendage of Hudson Bay. It is about 160 km wide between Pointe Louis-XIV on the east coast and Cape Henrietta Marie on the west. The Québec-Ontario border is at the bottom of the bay, and its numerous islands were administered by the Northwest Territories but are now part of the territory of Nunavut. Akimiski is the largest island with an area of 3001 km2.

The bay was discovered in 1610 by Henry Hudson and named for Thomas James, who entered the bay in 1631 and spent a difficult winter on Charlton Island. The bay was long a centre for trade as Cree brought furs down the many rivers draining the surrounding area. The major rivers, from east to west, are La Grande Rivière, Eastmain, Rupert, Nottaway, Harricana, Moose, Albany and Attawapiskat.

Fort Rupert, Moose Factory and Fort Albany - at the mouths of the rivers of the same names - were active Hudson's Bay Company posts. Moosonee, Ont, the largest settlement in the area, is the northern terminus of the Ontario Northland Railway. There are Indigenous settlements at Chisasibi (formerly Fort-George), Eastmain and Attawapiskat. Polar Bear Provincial Park (Ontario) borders on the northwest coast.

See also James Bay Project.