Third party is a term used in the United States that refers to a political party other than one of the two major parties, the Democratic Party and the Republican Party, that participate in national and state elections. Historically, since the coalition of organized parties in the 1830s, the U.S. has a two-party system. Following Duverger's law, the Electoral College with its "winner take all" award of electors in presidential elections has, over time, created the two-party system. Another contributing factor is the division of the government into three separate branches which differs from the parliamentary system.

Although third parties rarely win national elections, they can have an effect on them. Third parties can draw attention to issues that may be ignored by the majority parties. If the issue finds resonance with the voters, one or more of the major parties may adopt the issue into its own party platform. Also, a third party may be used by the voter to cast a protest vote as a form of referendum on an important issue. Third parties may also help voter turnout bringing more people to the polls. Third party candidates at the top of the ticket can help to draw attention to other party candidates down the ballot, helping them to win local or state office. In 2004 the U.S. electorate consisted of an estimated 43% registered Democrats and 33% registered Republicans, with independents and those belonging to other parties constituting 25%.