The list of Western Australia state by-elections includes every by-election held in the Australian state of Western Australia. By-elections occur whenever there is a vacancy in the Legislative Assembly (or, historically, the Legislative Council), although an imminent state election may allow the vacancy to remain until the dissolution of parliament.

Section 67 of the Electoral Act 1907 requires the Speaker to issue a writ for a by-election to fill the vacancy. This can either take place after a resolution of the House if Parliament is in session, or simply upon the cause being established by the Speaker acting alone if the Parliament is in adjournment for more than seven days.^[1]

A by-election occurs whenever there is a vacancy in the Legislative Assembly. Vacancies can occur for the following reasons, according to Section 38 of the Constitution Acts Amendment Act 1899.^[2]

The member:

• Is no longer eligible to sit—for example, leaving the State.^[3] This historically occurred more frequently when times were less economically stable, and people would move to another state or country where they could work or conduct business.
• Has been convicted for an offence for which the penalty was imprisonment for life, or for more than five years (Section 32). This last happened to John Marquis Hopkins when he was convicted for fraud in 1910, necessitating a by-election in his seat of Beverley.
• Becomes an undischarged bankrupt. This last happened in 1936 when Independent candidate Thomas Hughes won East Perth against the incumbent member in the 1936 election. However it was found that he was an undischarged bankrupt at the time of the nomination and poll (although had since resolved his affairs), and the election was declared void. He won the resulting by-election.
• Is elected to either the Federal Parliament or the parliament of another state or territory (Section 34). In practice, this is never necessary, as the other Parliament usually requires resignation from the Western Australian parliament in order to nominate as a candidate. This last occurred in the Legislative Assembly in 2001 when Hendy Cowan, the member for Merredin, resigned to unsuccessfully contest a seat in the Australian Senate.
• Is appointed as a judge or magistrate in a Western Australian court or as a chairman (or in some cases member) of another state board or tribunal (Section 34). This provision has never been acted upon, although numerous members have resigned over time to take up such an appointment—for example, Ron Davies resigned in Victoria Park in 1986 to become Agent-General for Western Australia in London, whilst Deputy Premier Herb Graham resigned in Balcatta in 1973 to become chairman of the Licensing Board.
• "Holds an office or place in the service of the Crown" at the time at which they take up their seat (Section 36-37). This has never been used at state level, although it has occurred in other States and in the Federal Parliament (for example, the circumstances leading to the federal Lindsay by-election in 1996.)
• Becomes "of unsound mind".
• Pledges allegiance to a foreign power after their election. This does not apply to members who are already dual citizens at the time of their election, unlike the equivalent section in the Federal Constitution.
• Fails to attend the House for one entire session without the permission of the House. The last time this caused a by-election was in 1915 when Joseph Gardiner, the Labor member for Roebourne, disappeared, necessitating a by-election in which an Opposition candidate was victorious, wiping out the Government's one-seat majority.

Additional reasons not within Section 38 include death, resignation, or the voiding of results by a Court of Disputed Returns.^[4] The last time a by-election took place for this reason was in 1983, when Gavan Troy, the Labor candidate, narrowly defeated incumbent Liberal member Tom Herzfeld in Mundaring. The seat was won by Troy by a larger margin at the resulting by-election.