Hudson Bay, sometimes called Hudson's Bay (usually historically), is a large body of saltwater in northeastern Canada with a surface area of 1,230,000 km² (470,000 sq mi). It is located north of Ontario, west of Quebec, northeast of Manitoba, and southeast of Nunavut, but politically entirely part of Nunavut. It is an inland marginal sea of the Arctic Ocean. The Hudson Strait provides a connection to the Labrador Sea and the Atlantic Ocean in the northeast, while the Foxe Channel connects Hudson Bay with the Arctic Ocean in the north. The Hudson Bay drainage basin drains a very large area, about 3,861,400 km² (1,490,900 sq mi), that includes parts of southeastern Nunavut, Alberta, Saskatchewan, Ontario, Quebec, all of Manitoba, and parts of the U.S. states of North Dakota, South Dakota, Minnesota, and Montana. Hudson Bay's southern arm is called James Bay.

The Eastern Cree name for Hudson and James Bay is Wînipekw (southern dialect) or Wînipâkw (northern dialect), meaning muddy or brackish water. Lake Winnipeg is similarly named by the local Cree, as is the location for the city of Winnipeg.

The bay is named after Henry Hudson, an Englishman sailing for the Dutch East India Company, and after whom the river that he explored in 1609 is also named. Hudson Bay encompasses 1,230,000 km² (470,000 sq mi), making it the second-largest water body using the term "bay" in the world (after the Bay of Bengal). The bay is relatively shallow and is considered an epicontinental sea, with an average depth of about 100 m (330 ft) (compared to 2,600 m (8,500 ft) in the Bay of Bengal). It is about 1,370 km (850 mi) long and 1,050 km (650 mi) wide. On the east it is connected with the Arctic Ocean (Davis Strait) by Hudson Strait; on the north, with the Arctic Ocean by Foxe Basin (which is not considered part of the bay), and Fury and Hecla Strait.

Hudson Bay is often considered part of the Arctic Ocean: the International Hydrographic Organization, in its 2002 working draft of Limits of Oceans and Seas, defined Hudson Bay, with its outlet extending from 62.5 to 66.5 degrees north (just a few miles south of the Arctic Circle) as being part of the Arctic Ocean, specifically "Arctic Ocean Subdivision 9.11". Other authorities include it in the Atlantic, in part because of its greater water budget connection with that ocean.

The search for a western route to Cathay and the East Indies, which had been actively pursued since the days of Christopher Columbus and John Cabot, in the latter part of the 15th century, directly resulted in the first sighting of Hudson Bay by Europeans. English explorers and colonists named Hudson Bay after Sir Henry Hudson who explored the bay beginning 2 August 1610, on his ship Discovery. On his fourth voyage to North America, Hudson worked his way around Greenland's west coast and into the bay, mapping much of its eastern coast. Discovery became trapped in the ice over the winter, and the crew survived onshore at the southern tip of James Bay. In the spring, as the ice receded, Henry Hudson expressed a desire to continue exploring the uncharted region. However, on 22 June 1611, the crew mutinied. They left Hudson and others adrift in a small boat. The fate of Hudson and the other men stranded with him remains undetermined. Nevertheless, there is little evidence in historical documents to suggest that they persisted for a long time thereafter. In May 1612, Sir Thomas Button sailed from England with two ships to look for Henry Hudson, and to continue the search for the Northwest Passage to Asia.

In 1668, Nonsuch reached the bay and traded for beaver pelts, leading to the creation of the Hudson's Bay Company (HBC), which still bears the historic name. The HBC negotiated a trading monopoly from the English Crown for the Hudson Bay watershed, called Rupert's Land. In 1670, the English Crown granted a charter to facilitate fur trading within the Hudson Bay drainage basin. France contested this grant by sending several military expeditions to the region, but abandoned its claim in the Treaty of Utrecht (April 1713).

The Treaty of Utrecht, signed on 11 April 1713, marked a significant agreement between Britain and France. The treaty was negotiated in Utrecht, Netherlands, and marked a crucial stage in the conclusion of the War of the Spanish Succession. Its provisions had a significant impact in shaping the postwar landscape and establishing a new order in both Europe and North America. French concessions in North America as outlined in the Treaty of Utrecht included: Hudson Bay region, Nova Scotia, and Newfoundland.

During this period, the HBC built several factories (forts and trading posts) along the coast at the mouth of the major rivers (such as Fort Severn; York Factory, Churchill; and the Prince of Wales Fort). The strategic locations were bases for inland exploration. More importantly, they were trading posts with Indigenous peoples who came to them with furs from their trapping season. The HBC shipped the furs to Europe and continued to use some of these posts well into the 20th century.^{[citation needed]}