Sir Joseph Cook, GCMG (7 December 1860 – 30 July 1947) was an Australian politician and the sixth Prime Minister of Australia. Born as Joseph Cooke and working in the coal mines of Silverdale, Staffordshire, during his early life, he emigrated to Lithgow, New South Wales, during the late 1880s, and became General Secretary of the Western Miners Association in 1887.

A founding member of the Australian Labor Party, Cook was elected to the New South Wales Legislative Assembly as Member for Hartley in 1891; the first Labor seat in any Australian parliament. Cook later switched to the Free Trade Party, and was a minister under George Reid from 1894 to 1899. During Australia's first federal election in 1901, Cook was elected unopposed to the federal seat of Parramatta, and served as the deputy to Reid, then Alfred Deakin following the creation of the Commonwealth Liberal Party from Cook's and Deakin's parties. As leader of the Liberal Party, Cook became Prime Minister following the 1913 elections, but as he only had a one-seat majority in the lower house and a minority in the upper house, he repeatedly sought the trigger for a double dissolution. The outbreak of World War I during the September 1914 election led to a Labor victory. Following a split in the Labor party in 1916, Cook joined Billy Hughes' Nationalist Party of Australia, and following the Nationalist victory in the 1917 election, served as Minister for the Navy, then Treasurer under Hughes. Cook retired from Parliament in 1921, and was appointed Australian High Commissioner in London. During 1928 and 1929, he headed the Royal Commission into South Australia as affected by Federation. He died in Sydney in 1947.

Cook was born as Joseph Cooke in Silverdale, a small mining town near Newcastle-under-Lyme in Staffordshire, England. He had no formal education and worked in the coal mines from the age of nine. During his teens he embraced Primitive Methodism, and marked his conversion by dropping the 'e' from his surname. He married Mary Turner in 1885 and shortly after emigrated to New South Wales.

Cook settled in Lithgow and worked in the coal mines, becoming General Secretary of the Western Miners Association in 1887. In 1888, he participated in demonstrations against Chinese immigration. He was also active in the Single Tax League and was a founding member of the Australian Labor Party in 1891.

Cook was elected to the New South Wales Legislative Assembly as MP for the coalfields seat of Hartley in 1891, in Labor's first big breakthrough in Australian politics. It was the first time Labor had won a seat in any parliament in Australia.

In 1894, however, Cook was the leader of the group who refused to accept the Labor Party's decision to make all members sign a "pledge" to be bound by decisions of the Parliamentary Labor Party (Caucus). He left the party and became a follower of George Reid's Free Trade Party. He was a minister in Reid's government from 1894 to 1899.

When the first federal Parliament was elected in 1901, Cook was elected, unopposed by Labor, member for Parramatta, a seat which then included the Lithgow area. He became Reid's deputy, but did not hold office in Reid's 1904 - 05 ministry, mainly because Reid needed to offer portfolios to independent Protectionist members. When Reid retired from the party leadership in 1908, Cook agreed to merge the Anti-Socialist Party (the Free Trade Party was renamed prior to the 1906 federal election) with Alfred Deakin's Protectionists, and became deputy leader of the new Commonwealth Liberal Party.

Cook served as Defence Minister in Deakin's 1909 - 1910 ministry, then succeeded Deakin as Liberal leader when the government was defeated by Labor in the 1910 elections. He had by this time become completely philosophically opposed to socialism.

At the 1913 elections Cook won a one-seat majority in the House of Representatives, while Labor retained a majority in the Senate, and in doing so became the 6th Prime Minister of Australia. Apparently unable to govern effectively without control of the Senate, Cook decided to bring about a double dissolution election under section 57 of the Constitution of Australia. He introduced a bill abolishing preferential employment for trade union members in the public service, a bill he knew the Senate would repeatedly reject. He then sought and obtained a double dissolution of the Parliament from the Governor General.

Unfortunately for Cook, World War I broke out in the middle of the election campaign for the September 1914 election. Fisher was able to remind the voters that it was Labor that had favoured an independent Australian defence force, which the conservatives had opposed. Cook was defeated and Fisher resumed office.

In 1916, the Labor government split when Fisher's successor, Billy Hughes, tried to introduce conscription. Cook agreed to become Hughes's deputy in the new Nationalist Party, and became Minister for the Navy in Hughes's government. The Nationalists had huge victories in the 1917 and 1919 elections. Cook was part of the Australian delegation at the Paris Peace Conference where he defended the White Australia Policy and supported Australia's annexation of German New Guinea. He was Treasurer (finance minister) 1920 - 21.

Cook resigned from Parliament in 1921 and was appointed Australian High Commissioner in London, where he served until 1927. During 1928 and 1929, he headed the Royal Commission into South Australia as affected by Federation. He died in Sydney in 1947, aged 86.

Cook was appointed to the Privy Council on 16 July 1914. He was knighted in 1918 as Knight Grand Cross of the Order of St Michael and St George (GCMG).

In 1972, he was honoured on a postage stamp bearing his portrait issued by Australia Post.