The Progressive Party was a third party in the United States formed in 1912 by former president Theodore Roosevelt after he lost the presidential nomination of the Republican Party to his former protégé rival, incumbent president William Howard Taft. The new party was known for taking advanced positions on progressive reforms and attracting leading national reformers. After the party's defeat in the 1912 presidential election, it went into rapid decline in elections until 1918, disappearing by 1920. The Progressive Party was popularly nicknamed the "Bull Moose Party" when Roosevelt boasted that he felt "strong as a bull moose" after losing the Republican nomination in June 1912 at the Chicago convention.As a member of the Republican Party, Roosevelt had served as president from 1901 to 1909, becoming increasingly progressive in the later years of his presidency. In the 1908 presidential election, Roosevelt helped ensure that he would be succeeded by Secretary of War Taft. Although Taft entered office determined to advance Roosevelt's Square Deal domestic agenda, he stumbled badly during the Payne–Aldrich Tariff Act debate and the Pinchot–Ballinger controversy. The political fallout of these events divided the Republican Party and alienated Roosevelt from his former friend. Progressive Republican leader Robert M. La Follette had already announced a challenge to Taft for the 1912 Republican nomination, but many of his supporters shifted to Roosevelt after the former president decided to seek a third presidential term, which was permissible under the Constitution prior to the ratification of the Twenty-second Amendment. At the 1912 Republican National Convention, Taft narrowly defeated Roosevelt for the party's presidential nomination. After the convention, Roosevelt, Frank Munsey, George Walbridge Perkins and other progressive Republicans established the Progressive Party and nominated a ticket of Roosevelt and Hiram Johnson of California at the 1912 Progressive National Convention. The new party attracted several Republican officeholders, although nearly all of them remained loyal to the Republican Party—in California, Johnson and the Progressives took control of the Republican Party.

The party's platform built on Roosevelt's Square Deal domestic program and called for several progressive reforms. The platform asserted that "to dissolve the unholy alliance between corrupt business and corrupt politics is the first task of the statesmanship of the day". Proposals on the platform included restrictions on campaign finance contributions, a reduction of the tariff and the establishment of a social insurance system, an eight-hour workday and women's suffrage. The party was split on the regulation of large corporations, with some party members disappointed that the platform did not contain a stronger call for "trust-busting". Party members also had different outlooks on foreign policy, with pacifists like Jane Addams opposing Roosevelt's call for a naval build-up.

In the 1912 election, Roosevelt won 27.4% of the popular vote compared to Taft's 23.2%, making Roosevelt the only third party presidential nominee to finish with a higher share of the popular vote than a major party's presidential nominee. Both Taft and Roosevelt finished behind Democratic nominee Woodrow Wilson, who won 41.8% of the popular vote and the vast majority of the electoral vote. The Progressives elected several Congressional and state legislative candidates, but the election was marked primarily by Democratic gains. The 1916 Progressive National Convention was held in conjunction with the 1916 Republican National Convention in hopes of reunifying the parties with Roosevelt as the presidential nominee of both parties. The Progressive Party collapsed after Roosevelt refused the Progressive nomination and insisted his supporters vote for Charles Evans Hughes, the moderately progressive Republican nominee. Most Progressives joined the Republican Party, but some converted to the Democratic Party and Progressives like Harold L. Ickes would play a role in President Franklin D. Roosevelt's administration. In 1924, La Follette set up another Progressive Party for his presidential run. A third Progressive Party was set up in 1948 for the presidential campaign of former vice president Henry A. Wallace.