Sir Billy Mackie Snedden, KCMG, QC (31 December 1926 – 26 June 1987) was an Australian politician who served as the leader of the Liberal Party from 1972 to 1975. He was also a cabinet minister from 1964 to 1972, and Speaker of the House of Representatives from 1976 to 1983.

Snedden was born in Perth, Western Australia. He served in the Royal Australian Air Force during World War II, and then studied law at the University of Western Australia. From 1951 to 1952, he was the inaugural federal chairman of the Young Liberal Movement. After a period working overseas for the Department of Immigration, Snedden returned to Australia in 1954 and settled in Melbourne. He was elected to the House of Representatives the following year, aged 28.

In 1964, Snedden was elevated to cabinet by Robert Menzies. He served as a government minister until the Liberal government's defeat at the 1972 election, under an additional four prime ministers. Snedden spent periods as Attorney-General (1964–1966), Minister for Immigration (1966–1969), Minister for Labour and National Service (1969–1971), and Treasurer (1971–1972). He was elected deputy leader of the Liberal Party in 1971, and replaced William McMahon as leader after the following year's election loss, thus becoming Leader of the Opposition.

Snedden led the Liberal Party to the 1974 federal election, which saw the Labor Party retain government with a narrow majority. Malcolm Fraser mounted two leadership challenges in early 1975, winning on the second attempt; by the end of the year he was prime minister. Snedden was elected to the speakership when the parliament next sat. He held the position for almost seven years, gaining a reputation for impartiality. In retirement, Snedden served as chairman of the Melbourne Football Club and on the board of the Victorian Football League. The unusual circumstances of his death in 1987, which involved the ex-girlfriend of his own son, attracted much public interest.

Snedden was born on 31 December 1926 at a private hospital in Newcastle Street, West Perth, Western Australia. He was the youngest of six children born to Catherine (née Mackie) and Alan Snedden. His mother was 43 years old when he was born, and his siblings, one sister and four brothers, were much older than him.

Snedden's parents were born in Alva, Clackmannanshire, Scotland, and spoke "broad Scots". His grandparents were illiterate, and his parents had only limited schooling; they were both from coal-mining families. They immigrated to Australia in 1912 with their three oldest children, initially settling in the remote mining town of Meekatharra. The family moved to Perth in about 1921, after their house in Meekatharra burned down.

Snedden's father worked mostly as a stonemason, but also spent periods as a miner and general labourer when better work was not available. He reputedly left Scotland to escape a paternity suit, and had earlier been in trouble with the law for poaching. The family name was originally "Snaddon" (or "Snadon"), but was changed upon arrival in Australia.

Snedden grew up in Perth's inner north near the suburb of Highgate, living initially on Robinson Avenue and later on Bulwer Street. His father left home when he was about three or four years old, and they had only limited contact thereafter. His oldest brother Bob became the family's main breadwinner, while his mother worked as a laundress and his other siblings also found jobs. He was close to his sister Jean, who often babysat him. Snedden worked part-time from about the age of eight, helping his brothers on construction sites and selling and delivering newspapers. He began his schooling at Highgate Primary School, and later attended the Perth Boys' School through to the ninth grade.