Joseph Aloysius Lyons (15 September 1879 – 7 April 1939) was an Australian politician who was the 10th Prime Minister of Australia, in office from 1932 until his death in 1939. He began his career in the Australian Labor Party (ALP), but became the founding leader of the United Australia Party (UAP) after the Australian Labor Party split of 1931. He had earlier been Premier of Tasmania from 1923 to 1928.

Lyons was born in Stanley, Tasmania, and before entering politics worked as a schoolteacher. He was active in the Labor Party from a young age and won election to the Tasmanian House of Assembly in 1909. He was Treasurer of Tasmania (1912–1914) under John Earle, before replacing Earle as party leader in 1916. After two elections that ended in hung parliaments, Lyons was appointed premier in 1923 at the head of a minority government. He pursued moderate reforms and successfully negotiated a constitutional crisis over the powers of the Legislative Council. At the 1925 election he led Labor to its first majority government in Tasmania, but the party lost office three years later.

In 1929, Lyons resigned from state parliament to enter federal politics, winning an Australian House of Representatives seat in Labor's landslide victory at the 1929 election. He was immediately appointed to cabinet by the new prime minister James Scullin, becoming Postmaster-General of Australia and Minister for Works and Railways. In 1930, he was acting treasurer while Scullin was overseas, and came into conflict with the Labor caucus over the government's response to the Great Depression; he preferred orthodox financial policies. In early 1931, Lyons and his followers left Labor to sit as independents. His exact motivations for leaving the party have been subject to debate. A few months later his group merged with other opposition parties to form the United Australia Party; he was elected Leader of the Opposition.

Lyons led the UAP to a landslide victory at the 1931 election. Nicknamed "Honest Joe", he was known as a masterful political campaigner and became popular with the general public. His personal popularity was a major factor in the government's re-election in 1934 and 1937; he was the first prime minister to win three federal elections. The UAP initially governed alone but after 1934 formed a coalition with the Country Party. Lyons was his own treasurer until 1935 and oversaw Australia's recovery from the Great Depression. He faced a number of foreign-policy challenges, but accelerated Australia's transition towards an independent foreign policy. In the lead-up to World War II his government pursued a policy of appeasement and rearmament.

Lyons died of a heart attack in April 1939, becoming the first Australian prime minister to die in office. He is the only prime minister from Tasmania and one of two state premiers who have become prime minister, along with George Reid. Several years after his death, his widow Enid Lyons became the first woman elected to the House of Representatives.