William Morris Hughes (25 September 1862 28 October 1952) was an Australian politician who served as the seventh prime minister of Australia, in office from 1915 to 1923. He is best known for leading the country during World War I, but his influence on national politics spanned several decades. Hughes was a member of federal parliament from Federation in 1901 until his death, the only person to have served for more than 50 years. He represented six political parties during his career, leading five, outlasting four, and being expelled from three.

Hughes was born in London to Welsh parents. He emigrated to Australia at the age of 22, and became involved in the fledgling Australian labour movement. He was elected to the New South Wales Legislative Assembly in 1894, as a member of the New South Wales Labor Party, and then transferred to the new federal parliament in 1901. Hughes combined his early political career with part-time legal studies, and was called to the bar in 1903. He first entered cabinet in 1904, in the short-lived Watson Government, and was later the Attorney-General of Australia in each of Andrew Fisher's governments. He was elected deputy leader of the Australian Labor Party in 1914.Hughes became prime minister in October 1915, when Fisher retired due to ill health. The war was the dominant issue of the time, and his support for sending conscripted troops overseas caused a split within Labor ranks. Hughes and his supporters were expelled from the party in November 1916, but he was able to remain in power at the head of the new National Labor Party, which after a few months merged with the Liberals to form the Nationalist Party of Australia. His government was re-elected with large majorities at the 1917 and 1919 elections. Hughes established the forerunners of the Australian Federal Police and the CSIRO during the war, and also created a number of new state-owned enterprises to aid the post-war economy. He made a significant impression on other world leaders at the 1919 Paris Peace Conference, where he secured Australian control of the former German New Guinea.

At the 1922 Australian federal election, the Nationalists lost their majority in parliament and were forced to form a coalition with the Country Party. Hughes's resignation was the price for Country Party support, and he was succeeded as prime minister by Stanley Bruce. He became one of Bruce's leading critics over time, and in 1928, following a dispute over industrial relations, he and his supporters crossed the floor on a confidence motion and brought down the government. After a period as an independent, Hughes formed his own organisation, the Australian Party, which in 1931 merged into the new United Australia Party (UAP). He returned to cabinet in 1934, and became known for his prescient warnings against Japanese imperialism. As late as 1939, he missed out on a second stint as prime minister by only a handful of votes, losing the 1939 United Australia Party leadership election to Robert Menzies.

Hughes is generally acknowledged as one of the most influential Australian politicians of the 20th century. He was a controversial figure throughout his lifetime, and his legacy continues to be debated by historians. His strong views and abrasive manner meant he frequently made political enemies, often from within his own parties. Hughes's opponents accused him of engaging in authoritarianism and populism, as well as inflaming sectarianism; his use of the War Precautions Act 1914 was particularly controversial. His former colleagues in the Labor Party considered him a traitor, while conservatives were suspicious of what they viewed as his socialist economic policies. He was extremely popular among the general public, particularly ex-servicemen, who affectionately nicknamed him "the little digger".

The prime minister of Australia is the head of government of the Commonwealth of Australia. The prime minister heads the executive branch of the federal government of Australia and is also accountable to federal parliament under the principles of responsible government. The incumbent prime minister is Scott Morrison, who took office in 2018 as leader of the Liberal Party.

Formally appointed by the governor-general, the office of the prime minister is governed by Westminster system convention as it is not described in the Australian constitution. To become prime minister, a politician should be able to command the confidence of the House of Representatives. As such, the prime minister is typically the leader of the majority party or coalition. Prime ministers do not have a set duration or number of terms, but an individual's term generally ends when their political party loses a federal election, or they lose or relinquish the leadership of their party.

Executive power is formally vested in the monarch and exercised by the governor-general on advice from government ministers, who are nominated by the prime minister and form the Federal Executive Council. The most senior ministers form the federal cabinet, which the prime minister chairs. The prime minister also heads the National Cabinet and the National Security Committee. Administrative support is provided by the Department of the Prime Minister and Cabinet. The prime minister has two official residences: The Lodge in Canberra and Kirribilli House in Sydney, as well as an office at Parliament House.

Thirty people have served as prime minister, the first of whom was Edmund Barton taking office on 1 January 1901 following federation. The longest-serving prime minister was Robert Menzies, who served over 18 years, and the shortest-serving was Frank Forde, who served one week. There is no legislated line of succession, however convention determines that the governor-general shall commission the deputy prime minister on a caretaker basis in the event of a vacancy.