Hudson Bay, large gulf or inland sea in the Keewatin and Baffin regions of the Nunavut Territory, east central Canada. It may be considered an arm of either the Atlantic Ocean, with which it is connected via Hudson Strait, or the Arctic Ocean, with which it is linked via Foxe Basin. Hudson Bay, along with its southern extension, James Bay, is about 1450 km (about 900 mi) long and up to about 965 km (about 600 mi) wide; it has an area of about 1,230,000 sq km (about 475,000 sq mi). The bay is bordered by Québec on the east, by Ontario on the south, and by Manitoba and Keewatin District on the west. Its northern end has several large islands, notably Southampton Island (41,214 sq km/15,913 sq mi), Coats Island (5499 sq km/2123 sq mi), and Mansel Island (3181 sq km/1228 sq mi). Another big island, Akimiski Island, is in James Bay, and many smaller islands are located off the eastern shore. The bay is relatively shallow. The average depth of Hudson Bay is about 101 m (about 330 ft), and its maximum depth is about 867 m (about 2,846 ft).

Formed by glaciation during the Pleistocene Epoch, Hudson Bay occupies a large depression in a vast area of ancient rocks called the Canadian Shield. The eastern coast of the bay is elevated and rocky, with some sheer cliffs, but its other shores are generally low-lying and, in places, marshy. Trees are present only around James Bay. The bay receives several major rivers, including the Churchill, Nelson, and Severn, and it drains a vast basin of about 3,846,000 sq km (about 1,485,000 sq mi). Because of the large inflow of fresh water, the bay has a relatively low level of salinity. It is navigable from early July to October, when it becomes clogged with ice. Nevertheless, much grain produced in the Prairie Provinces is shipped from Churchill, Manitoba, a port and railroad terminus at the mouth of the Churchill River.

The shores of Hudson Bay are sparsely settled, although small trading villages are located near the mouths of many of the rivers entering the bay in Québec, Ontario, and Manitoba. Indigenous peoples and Inuit are the chief inhabitants of the region; they support themselves principally by hunting and fishing. The bay has large numbers of fish, especially plaice, cod, halibut, and salmon. Whales, dolphins, walruses, and seals also inhabit its waters, and caribou are found on its shores. Numerous types of birds, such as ducks, geese, loons, gulls, and ptarmigan, live on the rocky coasts and islands of the bay.

The first European to explore the bay in 1610 was the English navigator Henry Hudson. The bay is named for him. Under the terms of a charter issued by Charles II of England, the Hudson's Bay Company held exclusive trading rights in the region of Hudson Bay from 1670 to 1870.