The supplementary vote (SV) is an electoral system used to elect a single winner, in which the voter ranks the candidates in order of preference. In an election, if no candidate receives an absolute majority of first preference votes, then all but the two leading candidates are eliminated and there is a second count. In the second count, the votes of those who supported eliminated candidates are distributed among the two remaining candidates, so that one candidate achieves an absolute majority.

It is a specific implementation of the contingent vote and instant-runoff voting. Under supplementary vote, voters express first and second ranked choices of candidate only, and, if no candidate receives an absolute majority of first-choice votes, all but the two leading candidates are eliminated. If a voter's first-choice candidate is eliminated but their second-choice is one of the two remaining candidates then their vote is transferred to the second-choice candidate. The candidate with the most votes is the winner. The supplementary vote was used in all elections for directly elected mayors in England, including the Mayor of London, and in elections for police and crime commissioners until 2022 when it was replaced by first-past-the-post voting (FPTP).