Alexander John Gosse Downer (born 9 September 1951) is an Australian former politician and diplomat who served as the Minister for Foreign Affairs from 1996 to 2007, and was a member of parliament (MP) for the South Australian division of Mayo from 1984 to 2008.

Downer was born in Adelaide, the son of Sir Alick Downer and the grandson of Sir John Downer. After periods working for the Bank of New South Wales and with the diplomatic service, he was appointed executive director of the Australian Chamber of Commerce in 1983. He also served as an advisor to Liberal leaders Malcolm Fraser and Andrew Peacock. Downer was elected to parliament at the 1984 federal election, winning the Division of Mayo in South Australia. He was added to the opposition frontbench in 1987.

After the Coalition lost the 1993 election, John Hewson's position as leader of the Liberal Party came into question. Downer successfully challenged him for the leadership in 1994, thus becoming Leader of the Opposition. He initially had high approval ratings, but after a series of gaffes, resigned from the leadership in 1995 and was replaced by John Howard. He was the first Liberal leader to fail to lead the party to an election, and remains the shortest-serving leader in party history.

When the Howard government came to power in 1996, Downer was made Minister for Foreign Affairs. He served until the government's defeat in 2007, making him the longest-serving foreign minister in Australian history. Downer left politics in 2008, and was subsequently named Special Adviser to the UN Secretary-General on Cyprus. He held that post until 2014, when he was appointed High Commissioner to the United Kingdom by the Abbott government.

The Downer family has a long history in South Australian politics. Their earliest ancestors were Mary Ann Downer (1792–1868) and her son Henry Downer (1811–1870), his great-grandfather, who travelled from England in 1862 aboard the Eden, settling in Adelaide. Downer's father was Minister for Immigration in the Menzies government and later served as High Commissioner to the United Kingdom. His paternal grandfather, Sir John Downer, served twice as Premier of South Australia in the late 19th century, and was later one of the inaugural members of the Australian Senate. Downer's maternal grandfather was the businessman and philanthropist James Hay Gosse, whose father was the explorer William Gosse.

Alexander Downer was born on 9 September 1951 in Adelaide, South Australia, when the family were living in the Adelaide Hills home built by his father, Arbury Park. He is the third of four children born to Sir Alexander "Alick" Downer and Mary Downer (née Gosse); he has three sisters.^{[citation needed]}

Downer was educated at Geelong Grammar School in Australia, then in England (while his father was High Commissioner) at Radley College between 1964 and 1970. He subsequently completed a Bachelor of Arts in politics and economics at Newcastle University in Newcastle upon Tyne.

From 1975 to 1976, he worked as an economist for the Bank of New South Wales, before entering the Australian Diplomatic Service, where he served until 1982. Some of Downer's time in the Diplomatic Service was spent at a posting in Brussels, where he undertook a French language training course. He then worked as an adviser to the then Liberal Prime Minister, Malcolm Fraser and subsequent Leader of the Federal Opposition Andrew Peacock.