A United States presidential nominating convention is a political convention held every four years in the United States by most of the political parties who will be fielding nominees in the upcoming U.S. presidential election. The formal purpose of such a convention is to select the party's nominees for popular election as President and Vice President, as well as to adopt a statement of party principles and goals known as the party platform and adopt the rules for the party's activities, including the presidential nominating process for the next election cycle. Conventions remain an important part of the political process despite the nominees almost always being determined during the primary season, as they provide positive publicity for the nominee and party, which can then lead to a convention bounce.

The earliest public national presidential nominating Conventions have been traced back to the 1832 election, before which smaller groups of party leaders chose the nominee, arguably beginning with the 1796 election. Since 1972, most of the delegates have been selected in presidential primaries state by state. Other delegates to these conventions include political party members who are seated automatically, and are called "unpledged delegates" because they can choose for themselves for which candidate they vote. The pledged delegates determined by the primaries generally allow the nominees to be decided before the convention opens, but if no single candidate has secured a majority of both pledged and unpledged delegates then a "brokered convention" can result.

In addition to the two major parties' quadrennial events: the Democratic and Republican National Convention, some minor parties also select their nominees by convention, including the Green Party, the Socialist Party USA, the Libertarian Party, the Constitution Party, and the Reform Party USA.

The convention cycle begins with the Call to Convention. Usually issued about 18 months in advance, the Call is an invitation from the national party to the state and territory parties to convene to select a presidential nominee. It also sets out the number of delegates to be awarded to each, as well as the rules for the nomination process. Since 1964, the conventions are usually scheduled for four days of business, with the exception of the 1972 Republican and 2012 Democratic conventions, which were scheduled for three days each. (The 2008 and 2012 Republican conventions were also three days each, but in each case was shortened from the scheduled four days due to weather issues.)

Since 1936 the party to which the incumbent president belongs has held its convention second. Between 1864 and 1952, the Democrats went second every year (except for 1888). In 1956, when Republican Dwight D. Eisenhower was the incumbent, the Democrats went first for the first time since 1888. So from 1936 to 1952, during administrations led by Democratic presidents Franklin D. Roosevelt and Harry S. Truman, the Democrats had their convention after the Republicans, but it is unclear whether they went second because they held the White House or because they had almost always gone second. 1956 became the first year it was clear a party went second because they were the party of the incumbent, and that protocol has been followed ever since.

Major party conventions from the start through 1948 were mostly held in May and June, with a few exceptions. This might have been due to the lack of air conditioning – the last conventions held without air conditioning were in 1948. Since 1952, all major party conventions have been held in the months of July, August or (for the first time in 2004), finishing in early September. Between the middle of the 20th century and 2004, the two major party conventions were primarily scheduled about one month apart, often with the Summer Olympics in between so they did not have to compete for viewers. In 1996, both were held in August to accommodate the Atlanta Olympics in July, the last Summer Olympics to date to be hosted in the U.S. In 2000, both conventions preceded the Sydney Olympics in late September.

In 2008 and 2012, the Democratic and Republican conventions were scheduled for back-to-back weeks following the conclusion of the Beijing and London Olympics, respectively, resulting in the later conventions starting in September for the first time (Republicans in 2008, Democrats in 2012, with the Democratic start on September 4, 2012, the latest any major convention has started). One reason for these late conventions had to do with campaign finance laws, which allow the candidates to spend an unlimited amount of money before the convention, but forbid fundraising after the convention, for the parties to receive federal campaign funds. However, moving the conventions into early September led to conflicts with the NFL Kickoff game, which is usually held on the first Thursday after Labor Day in September. (The NFL accommodated the conventions and moved its game to an earlier start time in 2008, and to a rare Wednesday in 2012. Convention dates in 2004, and 2016 through 2024 did not conflict with the NFL schedule). Additionally, election laws in some states would likely prevent conventions from moving later into mid-September. Ohio election laws forced the Democrats to schedule a virtual nomination of Kamala Harris prior to the scheduled 2024 convention, to get around an August 7 ballot deadline. (Such deadlines have been waived in one way or another in previous election cycles).

However, Barack Obama's choice not to receive federal campaign funds for the 2008 general election started a trend, and so the campaign finance reason for the late scheduling of conventions has gone away in subsequent cycles.