A faithless elector is a member of an electoral college who does not cast his/her electoral vote for the person whom they have pledged to elect. Only the United States allows such a system. .

On 158 occasions, electors have cast their votes for President or Vice President in a manner different from that prescribed by the legislature of the state they represented. Of those, 71 votes were changed because the original candidate died before the elector was able to cast a vote. Two votes were not cast at all when electors chose to abstain from casting their electoral vote for any candidate. The remaining 85 were changed by the elector's personal interest, or perhaps by accident. Usually, the faithless electors act alone. An exception was in 1836 when 23 Virginia electors changed their vote together.

Political parties choose their slate of electors in each state, and they generally select party members with a reputation for high loyalty to the party and its candidate. Moreover, a faithless elector runs a risk of censure and other political retaliation from his party. Thus, the parties have generally been successful in keeping their electors faithful, leaving out the cases in which a candidate died before the elector was able to cast a vote.

Twenty-four states have laws to punish faithless electors. While no faithless elector has ever been punished, the constitutionality of state pledge laws was brought before the Supreme Court in 1952 (Ray v. Blair, 343 U.S. 214). The court ruled in favor of the state's right to require electors to pledge to vote for their party's nominee, as well as to remove electors who refuse to pledge. Once the elector has voted, their vote can only be changed in states such as Michigan and Minnesota, where votes other than those pledged are rendered invalid. However, in all twenty-four states, a faithless elector may only be punished after he or she votes. As stated in the ruling, electors are acting as a function of the state, not the federal government. Therefore, states have the right to govern electors. The constitutionality of state laws punishing electors for actually casting a faithless vote, rather than refusing to pledge, has never been decided by the Supreme Court.

To date, faithless electors have never changed the otherwise expected outcome of the election.