The Justicialist Party (Spanish: Partido Justicialista, IPA: [paɾˈtiðo xustisjaˈlista]; abbr. PJ), also known as the Peronist Party (Spanish: Partido Peronista), is a major political party in Argentina, and the largest branch within Peronism. Following the 2023 presidential election, it has been the largest party in the opposition against President Javier Milei.

Founded by Juan Perón and his wife, First Lady Eva Perón, the party followed a left-wing agenda based on his policies. It is overall the largest party in Congress. Historically, the party's factual position was undermined by divisions that emerged in the 1990s and lasted until 2020; the PJ was rocked by a conflict between two Peronist tendencies, Kirchnerism, the main, left-wing populist faction, and Federal Peronism, which was located on the centre and centre-right of the political spectrum. The division ended with the failure of Federal Peronism to challenge the dominating Kirchnerist faction in 2019. This was completed by Cristina Kirchner, the leader of Kirchnerism, being elected the leader of the party, and the creation of a separate dissident party — the Federal Consensus. Today, the party adheres to an ideology based on economic intervention, welfare-state policies, and economic independence from wealthier countries; it is located on the centre-left and left wing of the political spectrum.

Aside from Juan Perón, who governed Argentina on three occasions from 1946 to 1955 and later from 1973 to 1974, eleven presidents of Argentina have belonged to the Justicialist Party: Héctor Cámpora, Raúl Alberto Lastiri, Isabel Perón, Carlos Menem, Ramón Puerta, Adolfo Rodríguez Saá, Eduardo Camaño, Eduardo Duhalde, Néstor Kirchner, Cristina Fernández de Kirchner and Alberto Fernández. Justicialists have been the largest party in Congress almost consistently since 1987.

The Justicialist Party was founded in 1946 by Juan and Eva Perón, uniting the Labour Party, the Radical Civic Union Renewal Board and the Independent Party, the three parties that had supported Peron in the election. After the enactment of women's suffrage, the Female Peronist Party, led by the First Lady, was also established. All Peronist entities were banned from elections after 1955, when the Revolución Libertadora overthrew Perón, and civilian governments' attempt to lift Peronism's ban from legislative and local elections in 1962 and 1965 resulted in military coups.

Basing itself on the policies espoused by Perón as Argentine president, the party's platform has, from its inception, centered on populism, and its most consistent base of support has historically been the General Confederation of Labor, Argentina's largest trade union. Perón ordered the mass nationalization of public services, strategic industries, and the critical farm export sector; enacted progressive labor laws and social reforms; and accelerated public works investment.

His tenure also favored technical schools, harassed university staff, and promoted urbanization as it raised taxes on the agrarian sector. Those trends earned Peronism the loyalty of much of the working and lower classes but helped alienate the upper and middle classes of society. Censorship and repression intensified, and following his loss of support from the influential Argentine Catholic Church, Perón was ultimately deposed in a violent 1955 coup.

The alignment of groups as supporting or opposing Peronism has largely endured, but the policies of Peronism itself varied greatly over the subsequent decades, as did increasingly those put forth by its many competing figures. During Perón's exile, it became a big tent party united almost solely by its support for the aging leader's return. A series of violent incidents, as well as Perón's negotiations with both the military regime and diverse political factions, helped lead to his return to Argentina in 1973 and to his election in September that year.

An impasse followed in which the party had a place both for leftist armed organizations such as Montoneros, and far-right factions such as José López Rega's Argentine Anti-Communist Alliance. Following Perón's death in 1974, however, the tenuous understanding disintegrated, and a wave of political violence ensued, ultimately resulting in the March 1976 coup. The Dirty War of the late 1970s, which cost hundreds of Peronists (among thousands more) their lives, solidified the party's populist outlook, particularly following the failure of conservative Economy Minister José Alfredo Martínez de Hoz's free trade and deregulatory policies after 1980.