Third party, or minor party, is a term used in the United States' two-party system for political parties other than the Democratic and Republican parties. The Electoral College for presidential elections and the plurality voting system for most other elections have established a two-party system in American politics. Third parties are most often encountered in presidential elections and while third-party candidates rarely win elections, they can have an effect on them through vote splitting and other impacts.

With few exceptions, the U.S. system has two major parties which have won, on average, 98% of all state and federal seats. According to Duverger's law two main political parties emerge in political systems with plurality voting in single-member districts. In this case, votes for minor parties can potentially be regarded as splitting votes away from the most similar major party. Third party vote splitting exceeded a president's margin of victory in three elections: 1844, 2000, and 2016.

There have only been a few rare elections where a minor party was competitive with the major parties, occasionally replacing one of the major parties in the 19th century. No third-party candidate has won the presidency since the Republican Party became the second major party in 1856 and won in 1860. Since then, a third-party candidate won states in five elections: 1892, 1912, 1924, 1948, and 1968. 1992 was the last time a third-party candidate placed second in any state, and 1996 was the last time a third-party candidate got over 5% of the vote nationally.

Greens, Libertarians, and others have elected state legislators and local officials. The Socialist Party elected hundreds of local officials in 169 cities in 33 states by 1912, including Milwaukee, Wisconsin; New Haven, Connecticut; Reading, Pennsylvania; and Schenectady, New York. There have been governors elected as independents, and from such parties as Progressive, Reform, Farmer-Labor, Populist, and Prohibition. After losing a Republican primary in 2010, Bill Walker of Alaska won a single term in 2014 as an independent by joining forces with the Democratic nominee. In 1998, wrestler Jesse Ventura was elected governor of Minnesota on the Reform Party ticket.

Sometimes a national officeholder that is not a member of any party is elected. Previously, Senator Lisa Murkowski won re-election in 2010 as a write-in candidate after losing the Republican primary to a Tea party candidate, and Senator Joe Lieberman ran and won reelection to the Senate as an "Independent Democrat" in 2006 after losing the Democratic primary. As of 2025, there are only two U.S. senators, Angus King and Bernie Sanders, who identify as Independent and both caucus with the Democrats.

The last time a third-party candidate carried any states in a presidential race was George Wallace in 1968, while the last third-party candidate to finish runner-up or greater was former president Teddy Roosevelt's 2nd-place finish on the Bull Moose Party ticket in 1912. The only three U.S. presidents without a major party affiliation upon election were George Washington, John Tyler, and Andrew Johnson, and only Washington served his entire tenure as an independent. Neither of the other two were ever elected president in their own right, both being vice presidents who ascended to office upon the death of the president, and both became independents because they were unpopular with their parties. John Tyler was elected on the Whig ticket in 1840 with William Henry Harrison, but was expelled by his own party. Johnson was the running mate for Abraham Lincoln, who was reelected on the National Union ticket in 1864; it was a temporary name for the Republican Party.

Electoral fusion in the United States is an arrangement where two or more United States political parties on a ballot list the same candidate, allowing that candidate to receive votes on multiple party lines in the same election.

Electoral fusion is also known as fusion voting, cross endorsement, multiple party nomination, multi-party nomination, plural nomination and ballot freedom.