The Republican Party, also referred to as the GOP ("Grand Old Party"), is one of the two major contemporary political parties in the United States, along with its main historic rival, the Democratic Party.

The GOP was founded in 1854 by opponents of the Kansas–Nebraska Act, which allowed for the potential expansion of chattel slavery into the western territories. It was simultaneously strengthened by the collapse of the Whig Party, which had previously been one of the two major parties in the country. Upon founding, the Republican party supported economic reform and classical liberalism while opposing the expansion of slavery. It consisted of northern Protestants, factory workers, professionals, businessmen, prosperous farmers, and after 1866, former black slaves. The Republican Party had almost no presence in the Southern United States at its inception, but was very successful in the Northern United States, where by 1858 it had enlisted former Whigs and former Free Soil Democrats to form majorities in nearly every Northern state. While both parties adopted pro-business policies in the 19th century, the early GOP was distinguished by its support for the national banking system, the gold standard, railroads, and high tariffs.

Beginning with the first Republican President, Abraham Lincoln, the party largely dominated the national political scene until 1932. Under the leadership of Lincoln and a Republican Congress, it led the fight to destroy the Confederate States of America during the American Civil War; slavery was banned in the United States in 1865. In 1912, former Republican president Theodore Roosevelt formed the Progressive ("Bull Moose") Party after being rejected by the GOP and ran unsuccessfully as a third-party presidential candidate calling for social reforms. After 1912, many Roosevelt supporters left the Republican Party, and the Party underwent an ideological shift to the right. The GOP lost its congressional majorities during the Great Depression (1929–1940); under President Franklin D. Roosevelt, the Democrats formed a winning New Deal coalition that was dominant from 1932 through 1964.

Following the Civil Rights Act of 1964 and the Voting Rights Act of 1965, the party's core base shifted, with southern states becoming more reliably Republican in presidential politics. After the Supreme Court's 1973 decision in Roe v. Wade, the Republican Party opposed abortion in its party platform and grew its support among evangelicals. Its 21st-century ideology is American conservatism, with the party supporting free market economics, social conservatism, and originalism in constitutional jurisprudence. The GOP supports lower taxes, deregulation, increased military spending, restrictions on abortion, restrictions on immigration, gun rights, and restrictions on labor unions. It was strongly committed to protectionism and tariffs at its founding, but grew more supportive of free trade in the 20th century.In the 21st century, the demographic base skews toward men, people living in rural areas, people living in the American South, and white Americans, particularly white evangelical Christians. Its most recent presidential nominee was Donald Trump, who served as the 45th president of the United States from 2017 to 2021. There have been 19 Republican presidents, the most from any one political party. As of early 2022, the GOP controls 28 state governorships, 30 state legislatures, and 23 state government trifectas. Six of the nine sitting U.S. Supreme Court justices were nominated by Republican presidents.