Juan Bautista Justo (June 28, 1865, in Buenos Aires – January 8, 1928, in Buenos Aires) was an Argentine physician, journalist, politician, and writer. After finishing medical school he joined the Civic Union of the Youth, later participating in the foundation of the Socialist Party in 1896, of which he was chief director until his death. He married Alicia Moreau de Justo, with whom he had three children.

Justo studied medicine at the Universidad de Buenos Aires while working as a journalist to earn a living. He graduated with honors in 1888, after which he travelled to Europe where he learned about socialist ideas. Back in Argentina, he worked as a surgeon at the Hospital de Crónicos.

Justo wrote his first pieces at the La Prensa newspaper as a parliamentary journalist. At the beginning of 1890 he started writing for the socialist newspaper El Obrero. In 1894, together with Augusto Kühn and Esteban Jiménez he founded the La Vanguardia publication that became two years later, with the creation of the Socialist Party, the official organ of the party. In 1905 La Vanguardia became a daily newspaper, in an important cultural extension that exceeded its original purpose of political diffusion. Justo directed the newspaper until his death.

Juan B. Justo participated in the UCR from its beginnings as Unión Cívica in 1889. During the Park Revolution of 1890 he took care of the revolutionary wounded. Shortly after, Justo deepened its participation in workers' circles and socialist currents, to finally found the Socialist Party in 1896 with Estéban Jiménez, Augusto Kühn and Isidoro Salomó. He directed the party from its creation to his death in 1928.

In 1912 Justo was elected a national deputy, seat that he kept until 1924 when he was elected senator, in both cases representing the city of Buenos Aires. As a deputy he was chief of the commission that investigated the trusts conflict, and participated of the deliberations that led to the University Reform of 1918. Justo also proposed several projects of law for the social welfare, and against gambling, alcoholism and illiteracy.

Justo also participated of the creation of cooperative El Hogar Obrero, the Biblioteca Obrera and the Sociedad Luz of culture and promotion of socialism. As president of the Argentine Socialist Party, he assisted to the socialist congresses held in Copenhagen and Bern.

Justo focused on political and social essays. His most important work is considered Theory and Practice of History (Teoría y práctica de la historia) of 1909. He also wrote Scientific Theory of History (Teoría científica de la historia) in 1898, Argentine Socialism (El socialismo argentino) in 1910, Socialism and Imperialism (Socialismo e imperialismo), The Socialist International (La internacional socialista), The Socialist Program for Agriculture (El programa socialista en el campo), and the posthumously published The Currency (La moneda) of 1937 and The Free Cooperation (La cooperación libre) of 1938. Justo also made the first Spanish translation from German of Karl Marx's Das Kapital.

• Biography at El Historiador