Sir Ninian Martin Stephen (15 June 1923 – 29 October 2017) was an English-born Australian judge who served as the 20th governor-general of Australia, in office from 1982 to 1989. He was previously a justice of the High Court of Australia from 1972 to 1982.

Stephen was born in England to Scottish parents. As a child he lived for periods in France, Germany, Scotland, and Switzerland, eventually arriving in Australia at the age of 16. Stephen served with the Australian Army during World War II, and after the war entered the legal profession. He became one of Australia's leading constitutional lawyers. Stephen was appointed to the Supreme Court of Victoria in 1970, and then to the High Court in 1972, aged 48. He was considered a member of the court's "moderate centre". In 1982, Stephen was appointed governor-general on the recommendation of Malcolm Fraser. He approved two double dissolutions during his time in office, the only governor-general to do so. After his term expired, Stephen remained active in public life as a conservationist and member of various international tribunals.

Stephen was born in Nettlebed, Oxfordshire, England, to Scottish parents, Barbara (née Cruickshank) and Frederick Brown Stephen. His father, a chauffeur, poultry farmer and motorcycle courier in World War I, left the family shortly after his birth, emigrating to Canada and starting a new family; his son was told that he had died, and did not learn the truth until 2003. Stephen's mother, formerly a lady's maid was a paid companion for Nina Mylne, the wealthy heiress of the Queensland pastoralist Graham Mylne; his given name was in her honour. During his early childhood, the three of them lived for periods in Switzerland (Geneva where he was christened, and Montreaux), France (Paris, Cannes, and Saint-Cast-le-Guildo) and Germany (Wiesbaden), where Mylne took him to Nuremberg for the 1938 Reichsparteitag Grossdeutschland (5-12 September) which he photographed. They eventually moved to Edinburgh in 1929 so Stephen could begin his formal schooling.

Mylne paid for Stephen's education, which took place in Scotland (George Watson's College and Edinburgh Academy), London (St Paul's School), and Switzerland (Chillon College, Montreux). He and Mylne generally traveled together, while his mother remained in Scotland and ran a boardinghouse. In 1940, the three of them moved to Melbourne to avoid the war, booking first into the Oriental Hotel then taking a flat in Linden Hall opposite Scotch College which Stephen attended for two terms, and was then accepted into the University of Melbourne to study law.

In December 1941, following the end of his first year at university, Stephen enlisted in the Citizens Military Force to serve part-time in the Melbourne University Regiment. Following Japan's entry into World War II, Stephen completed full-time military training from 8 December 1941 to 15 February 1942 and was then posted to the 10th Field Regiment, Royal Australian Artillery, serving in Western Australia. He subsequently transferred to the Second Australian Imperial Force. In late 1943, Stephen transferred to the Royal Australian Engineers, serving in New Guinea from April to August 1944 with the 43rd Australian Water Transport Company. He then attended courses, culminating in a commissioning course in April 1945. As a lieutenant, he served in the 41st Australian Landing Craft Company in New Guinea and New Britain. In August 1945, he was posted to Labuan, Borneo, arriving after the war's end and serving there until January 1946. After returning to Australia, he was discharged on 5 February 1946.

Stephen completed his studies after the war's end, and was admitted to the Victorian Bar in 1949. By the 1960s, he had become one of Australia's leading constitutional and commercial lawyers. He was made a Queen's Counsel in 1966.

On 30 June 1970, Stephen was appointed as a judge of the Supreme Court of Victoria. He held this position until 29 February 1972, relinquishing it to take up his appointment as a justice of the High Court of Australia. He was sworn of the Privy Council of the United Kingdom in 1979 and sat as a member of its Judicial Committee.

Although Stephen was appointed to the High Court by a Liberal government, he proved not to be a traditional conservative upholder of states' rights. He joined the "moderate centre" of the court, between the arch-conservatism of Sir Garfield Barwick and the radicalism of Lionel Murphy. In 1982 he was part of the majority that decided on a broad interpretation of the "external affairs power" of the Australian constitution in the Koowarta v Bjelke-Petersen case.