A nonpartisan primary, top-two primary, or jungle primary is a primary election in which all candidates for the same elected office run against each other at once, regardless of political party. This distinguishes them from partisan primaries, which are segregated by political party. This is the first round of a two-round system. As opposed to most two-round systems, the "first round" is a primary held before Election Day, and the "second round" is not optional (most two-round systems skip the second round if the winner of the first one gets more than 50%). A two-round system where the first round is held on Election Day is known in the US as runoff voting or top-two runoff.

Advocates claim two-round systems will elect more moderate candidates, as members of a minority party could vote for a more moderate candidate from the majority party, with some political scientists expressing similar views. However, empirical research on the system have found no effect on candidate moderation or turnout among independent voters. A first round with only two winners is also susceptible to vote-splitting: the more candidates from the same party run in the primary, the more likely that party is to lose.

The top-two system is used for all primaries in Washington and California (except presidential primaries). Alaska has used a highly-similar top-four primary with a ranked-choice runoff since the 2022 House special election.

The nonpartisan (jungle) primary is different from the partisan blanket primary. They are similar in that voters can vote in the first round for a candidate from any political party. The partisan blanket primary was used in Washington for nearly 65 years and briefly in California. However, the blanket primary was ruled unconstitutional in 2000 by the Supreme Court of the United States in California Democratic Party v. Jones, as it forced political parties to associate with candidates they did not endorse. The nonpartisan (jungle) primary disregards party preference in determining the candidates to advance to the general election, and for that reason, it was ruled facially constitutional by the Supreme Court in the 2008 decision Washington State Grange v. Washington State Republican Party.

Chief Justice John Roberts concurred in the 2008 decision, stating: "If the ballot is designed in such a manner that no reasonable voter would believe that the candidates listed there are nominees or members of, or otherwise associated with, the parties the candidates claimed to 'prefer', the I–872 primary system would likely pass constitutional muster." Each candidate for partisan office can state a political party that they prefer. Ballots must feature disclaimers that a candidate's preference does not imply the candidate is nominated or endorsed by the party or that the party approves of or associates with the candidate.

Subsequent as applied challenges were rejected by lower courts. On October 1, 2012, the US Supreme Court refused to hear appeals from Washington Libertarian Party and Washington State Democratic Party. The Washington State Republican Party had earlier dropped out of the appeal process.

Both Washington and California implement a two-winner nonpartisan primary by plurality vote.

The plan is used in Texas and other states in special elections but not primaries. A notable example involved former US Senator Phil Gramm, who in 1983 (while a member of the House of Representatives), after switching from the Democratic to the Republican Party, resigned his seat as a Democrat on January 5, ran as a Republican for his own vacancy in a special election held on February 12, and won rather handily.