Ian McCahon Sinclair AC (born 10 June 1929) is an Australian former politician who served as a Member of Parliament for 35 years, and was leader of the National Party from 1984 to 1989. He served as either a minister or opposition frontbencher for all but a few months from 1965 to 1989, and later Speaker of the House of Representatives from March to August 1998.

Sinclair was born in Sydney and studied law at the University of Sydney. He later bought a farming property near Tamworth. Sinclair was elected to parliament in 1963, and added to the ministry in 1965 as part of the Menzies Government. Over the following six years, he held various portfolios under Harold Holt, John McEwen, John Gorton and William McMahon. Sinclair was elected deputy leader of his party in 1971. He was a senior member of the Fraser government, spending periods as Minister for Primary Industry (1975–1979), Minister for Communications (1980–1982), and Minister for Defence (1982–1983). In 1984, Sinclair replaced Anthony as leader of the Nationals. He led the party to two federal elections, in 1984 and 1987, but was replaced by Charles Blunt in 1989. Sinclair was father of the parliament from 1990 until his retirement at the 1998 election. He spent his final six months in parliament as Speaker of the House of Representatives, following the sudden resignation of Bob Halverson; he is the only member of his party to have held the position. He also served as co-chair of the 1998 constitutional convention, alongside Barry Jones.

Sinclair is the earliest elected MP living (along with Senator Doug McClelland), the only surviving Country/Nationals MP elected in the 1960s. Additionally, he is the last surviving minister who served in the Menzies, Holt, McEwen, Gorton, and McMahon governments, as well as the First Fraser Ministry. He is entitled to the Right Honourable prefix as one of the few surviving Australian members of the Privy Council of the United Kingdom.

Sinclair was born in Sydney on 10 June 1929. He was the son of Gertrude Hazel (née Smith) and George McCahon Sinclair. His father was a chartered accountant who also served as deputy mayor of Ku-ring-gai Council, chairman of Knox Grammar School, and an elder of the Presbyterian Church.

Sinclair attended Knox Grammar before going on to the University of Sydney, where he graduated Bachelor of Arts in 1949 and Bachelor of Laws in 1952. He served in the No. 22 Squadron RAAF from 1950 to 1952, as part of the Citizen Air Force. Sinclair served his articles of clerkship with Norton Smith & Co., but did not pursue a legal career. He instead took up a grazing property near Bendemeer and set up the Sinclair Pastoral Company, of which he became managing director. He was a director of the Farmers and Graziers' Co-operative Limited from 1962 to 1965.

Sinclair married Margaret Anne Tarrant in 1956, with whom he had one son and two daughters. She died of brain cancer in December 1967. He remarried on 14 February 1970 to Rosemary Fenton, who had been Miss Australia in 1960; they had one son together. His daughter Fiona married Liberal politician Peter King.

A member of the Country Party, Sinclair was appointed to the New South Wales Legislative Council in 1961. He resigned in order to seek election to the House of Representatives at the 1963 federal election, retaining the Division of New England for the Country Party after the retirement of David Drummond.

In 1965, Sinclair was appointed Minister for Social Services in the Menzies Government, replacing Hugh Roberton. He stood for the deputy leadership of the Country Party after the 1966 federal election, but was defeated by Doug Anthony. In 1968, he became Minister for Shipping and Transport in the Gorton government. When Country Party leader John McEwen retired in February 1971, Anthony was elected as his replacement and Sinclair defeated Peter Nixon for the deputy leadership. He was appointed Minister for Primary Industry. A month later, William McMahon replaced John Gorton as Liberal leader and prime minister. McMahon wanted Sinclair to become Minister for Foreign Affairs, but for various reasons had to keep him in the primary industry portfolio and appoint Les Bury as foreign minister instead. Sinclair did later serve as acting foreign minister in Bury's absence.