Marc-Joseph Marion du Fresne (22 May 1724 – 12 June 1772) was a French privateer, East India captain, and explorer. The expedition he led to find the hypothetical Terra Australis in 1771 made important geographic discoveries in the south Indian Ocean and anthropological discoveries in Tasmania and New Zealand. In New Zealand, they spent longer living on shore than any previous European expedition. Half way through the expedition's stay, Marion was killed during a military assault by Ngare Raumati: one of the oldest Māori tribes from the Whangārei region.

He is commemorated with the toponyms Marion Island, South Africa and Marion Bay, Tasmania, as well in the name of two successive French oceanic research and supply vessel the Marion Dufresne (1972) and the Marion Dufresne II, which service the French Southern Territories of Amsterdam Island, the Crozet Islands, the Kerguelen Islands, and Saint Paul Island.

Marion was born in Saint Malo in 1724 into the, non-noble but wealthy, Marion family of shipowners and merchants. He eventually inherited a farm 'Le Fresne' near the village of Saint-Jean-sur-Vilaine and styled himself Marion Dufresne (or in some instances Dufresne-Marion). He was never known as (or signed himself) 'Du Fresne', but this has become a familiar appellation in New Zealand and Tasmania. He first went to sea in 1741 on a voyage to Cadiz aboard the 22-gun Saint-Ésprit.

During the War of the Austrian Succession, Marion commanded several ships as a privateer, including the Prince de Conty where he transported Charles Edward Stuart from Scotland to France. In the Seven Years' War, he was engaged in various naval operations including taking the astronomer Alexandre Guy Pingré to observe the 1761 transit of Venus in the Indian Ocean.

In January 1762, Marion received a grant of 625 argents of land at Quartier Militaire in Mauritius. Although he returned to France in 1764 and 1767, he made the island home in 1768.

In October 1770, Marion convinced Pierre Poivre, the civil administrator in Port Louis, to equip him with two ships and send him on a twofold mission to the Pacific. Marion's fellow explorer Louis Antoine de Bougainville had recently returned from the Pacific with a Tahitian native, Ahutoru. Marion was tasked with returning Ahutoru to his homeland, and then to explore the south Pacific for the hypothetical Terra Australis Incognita. For these purposes Marion was given two ships, the Mascarin and the Marquis de Castries and departed on 18 October 1771.

Marion spent most of his personal fortune on outfitting the expedition with supplies and a crew. He hoped to make a significant profit on the journey by trading with the reportedly wealthy islands of the South Pacific. No part of Marion's mission could be achieved; Ahutoru died of smallpox shortly after embarkation, and the expedition did not locate Terra Australis nor make a profit from trade. Instead, Marion discovered first the Prince Edward Islands and then the Crozet Islands before sailing towards New Zealand and Australia. His ships spent several days in Tasmania, where Marion Bay in the south-east is named after him. He was the first European to encounter the Aboriginal Tasmanians.

Marion sighted New Zealand's Mount Taranaki on 25 March 1772, and named the mountain Pic Mascarin without knowing that James Cook had named it "Mount Egmont" three years earlier.^{[citation needed]}