The 1918 Swan by-election was a by-election for the Division of Swan in the Australian House of Representatives, following the death of the sitting member Sir John Forrest. Held on 26 October 1918, the by-election led to the election of the youngest person to be elected until 2010 to the Parliament of Australia, Edwin Corboy. It saw the conservative vote split between the Country Party and the Nationalist Party, which directly prompted the introduction of preferential voting in Australia.

Sir John Forrest, who had been the first Premier of Western Australia, was elected to the Australian House of Representatives for the Division of Swan at the first federal election on 29 March 1901. Forrest then retained this seat until 2 September 1918, when he died off the coast of Sierra Leone from cancer while en route to England. Thus, a by-election was called to replace Forrest as the representative for Swan.

The by-election was the first of what have come to be known in Australia as "three-cornered contests" with two anti-Labor parties both running candidates against Labor. The Nationalist candidate was William Hedges; Basil Murray was the Country Party candidate; Edwin Corboy was the Labor candidate, with William Watson standing as an independent. Hedges was previously a member of the House of Representatives for the Division of Fremantle, from 1906 to 1913.^[2] Forrest had won Swan in the previous election unopposed.

Corboy, who was elected just 22 years and two months old, remained the youngest person ever to be elected to either house of the Parliament of Australia^[4][5] until the 2010 election, when 20-year-old Wyatt Roy was elected as a member of the Liberal National Party of Queensland to the seat of Longman.

Shocked by the loss of a safe Nationalist seat to Labor, the Nationalist government was moved to initiate electoral reform and change the voting system to preferential voting (also known outside Australia as instant-runoff voting) as part of a rewrite of the electoral legislation, with the Commonwealth Electoral Act 1918.^[6] While preferential voting had already been introduced at the state level in Western Australia (1907) and Victoria (1911) and had been considered at the federal level by Sir Joseph Cook's government (1913–1914), it was only the "considerations of partisan advantage [and not] the finer points of electoral theory" that provided the impetus for the change.^[7]

Preferential voting was first put to use in the Corangamite by-election for the Victorian seat of Corangamite two months later, and Labor candidate and future Prime Minister James Scullin topped the primary vote, only to be defeated after distribution of preferences by William Gibson of the Victorian Farmers' Union.^[1] The preferential voting system remains in place and has helped to support a fairly stable three-party system, with the anti-Labor parties (presently the Liberal Party of Australia and the National Party of Australia) regularly forming coalition governments.^[1]

At the 1919 election, Corboy once again polled the highest percentage at the first count, with two conservative candidates again splitting the conservative vote, but with the introduction of preferential voting, Corboy was easily defeated by the Country Party's John Prowse on preferences.^[8]

In a twist of fate William Watson, who finished a distant fourth in this by-election with just 4.6% of the vote, would be elected to the House of Representatives for the Division of Fremantle at the 1922 election on the back of preferences from Nationalist candidate William Hedges, the same man who had also contested the by-election and had previously been the member for Fremantle.^[9]

• List of Australian federal by-elections