CPO-STV, or the Comparison of Pairs of Outcomes by the Single Transferable Vote, is a ranked voting system designed to achieve proportional representation. It is a more sophisticated variant of the Single Transferable Vote (STV) system, designed to overcome some of that system's perceived shortcomings. It does this by incorporating some of the features of Condorcet's method, a voting system designed for single-winner elections, into STV. As in other forms of STV, in a CPO-STV election more than one candidate is elected and voters must rank candidates in order of preference. As of February 2021, it has not been used for a public election.

CPO-STV aims to overcome the problems of tactical voting in traditional forms of STV, where a candidate can be eliminated at an early stage in the process that might have gone on to be elected later had they been allowed to remain in the contest. CPO-STV works by an exhaustive comparison of the various possible outcomes of an election, in accordance with a particular procedure, in order to determine which outcome best matches the preferences of voters. If used for a single winner election, CPO-STV becomes the same as Condorcet's method, in the same way that traditional STV becomes instant-runoff voting (IRV). The system was invented by Nicolaus Tideman.