Thomas Simpson (2 July 1808 – 14 June 1840) was a Scottish Arctic explorer, fur trader and cousin of Governor Sir George Simpson of the Hudson's Bay Company (HBC). He is known for helping chart the northern coasts of Canada as well as his mysterious death by violence while traveling near the Turtle River in what was then part of the Territory of Iowa (now the U.S. state of North Dakota). The circumstances of Simpson's final hours—in which he allegedly killed himself after gunning down two companions—have long been a subject of controversy.

Thomas Simpson was born in Dingwall, Ross-shire, Scotland, the son of magistrate and schoolteacher Alexander Simpson (1751–1821) by his second wife Mary, who had helped raise Thomas' cousin Sir George Simpson. He had a half-brother, Aemilius, and a full brother, Alexander. Simpson was a sickly and timid youth, avoiding rough sport. After his father's death the family ended up in financial distress, but despite this he was given a proper education.

Simpson was educated with a view to his becoming a clergyman, and was sent to King's College, Aberdeen, at the age of 17. He performed quite well and had been given the Huttonian prize, the highest award at the college, by the end of his fourth year. George Simpson offered Thomas a position in the Hudson's Bay Company (HBC) in 1826, which he declined in order to complete his studies.

Simpson graduated in 1828, at the age of 20, with a Master of Arts degree. He enrolled in a divinity class that winter with the goal of becoming a clergyman when the offer of a position in the HBC was again extended; this time he accepted. By his own confession, Simpson had "a little of the spirit of contradiction and an unwillingness to be led." In 1829 he arrived in Norway House to join the HBC as George's secretary. He was quite ambitious, and in a letter to his brother stated that his talents would lead him to speedy advancement. Simpson was stationed at the Red River Colony in the 1830s, serving as second officer to chief factor Alexander Christie.

From 1836 to 1839, Simpson was involved in an expedition to chart the Arctic coast of Canada in order to fill two gaps left by other expeditions in search of the Northwest Passage. The expedition was headed by Peter Warren Dease, a chief factor of the HBC. Simpson was the junior officer, but Dease ceded most of the responsibility to him. Several writers present Simpson as an ambitious and over-confident young man, whereas Dease was twenty years older, experienced in Arctic exploration and efficient but perhaps under-confident. Simpson and Dease were accompanied by ten more men, including canoemen James McKay and George Sinclair.

The expedition was organized by the HBC rather than the British Royal Navy, which sponsored most expeditions pressing the Northwest Passage. They were to descend the Mackenzie River to the Arctic Ocean, turn west and close the gap between John Franklin's 1826 furthest-west and Frederick William Beechey's furthest-east at Point Barrow. The next summer they were to go down the Coppermine River, repeat Franklin's 1821 route east to Cape Turnagain and continue along the unknown coast at least to the mouth of the Back River, which had been reached overland in 1834. They spent the winter of 1836 at Fort Chipewyan, where they built two 24-foot (7.3 m) boats.

The party left on 1 June, and a month later reached the mouth of the Great Bear River. There they detached four men to go upriver to the Great Bear Lake and build winter quarters at Fort Confidence while the rest went down the Mackenzie to the Arctic, which they reached on 9 July. They then traveled west along the coast past Franklin's Return Reef until they were blocked by ice at Boat Extreme, about 50 miles (80 km) east of Point Barrow. Simpson and five men continued on foot and reached Point Barrow on 4 August. They returned to Fort Confidence on 25 September. At this point the north coast had been mapped from the Bering Strait to the mouth of the Coppermine.

Early in the year, Simpson went overland to find the upper Coppermine River. In the summer they descended the Coppermine, which was full of meltwater, and reached the still-frozen Arctic. They waited two weeks for the ice to clear and began working slowly east. On 20 August they were blocked by ice a few miles from Franklin's Point Turnagain on the Kent Peninsula. Dease stayed behind with the boats and Simpson walked about 100 miles (160 km) east to a place he called Point Alexander. To the north he saw and named Victoria Land. To the east he saw open water in Queen Maud Gulf. He returned to Dease and the frozen-in boats. A few days later the ice suddenly cleared and they had an easy sail back to the Coppermine. They had gone only a little further than Franklin.