Louis Francois Marie Aleno de Saint Aloüarn (25 July 1738 – 27 October 1772) was a notable French mariner and explorer.
St Aloüarn was the first European to make a formal claim of sovereignty — on behalf of France — over the west coast of Australia, which was known at the time as "New Holland". While indigenous Australians had lived there for thousands of years, the European empires of the early modern era frequently did not recognise the sovereignty of indigenous peoples.
Much of the west coast had already been charted by mariners from the Netherlands, following a landing by Dirk Hartog in 1616, who left a commemorative plaque recording his visit. James Cook, in 1770, had charted and claimed the east coast for Britain. When St Aloüarn visited New Holland in 1772, neither British nor Dutch officials had issued a formal claim over the western part of New Holland. However, the French claim over Western Australia was never secured by a permanent settlement.
St Aloüarn's parents were François Marie Guénolé Pantaléon d’Aleno and Marie Josèphe Pélagie de Quillien, both members of the . He was born at Saint-Aloüarn, near Guengat, Brittany.
The family, including St Aloüarn's father, a history of service in the French Navy and St Aloüarn joined the Gardes de la Marine in 1754. As a naval cadet, he joined his uncle, René de Rosmadec St Aloüarn, on the 74-gun warship Espérance. In November 1755, as it returned from a campaign off Canada, Espérance was attacked and captured by HMS Orford and HMS Revenge. St Aloüarn and his uncle became prisoners of war and were held in England for two years, before they were returned to France. Because of his bravery under fire, St Aloüarn was promoted to ensign.