Austrian Social Democrat, founder of a party representing all the nationalities of Austria-Hungary.

Born into a wealthy Jewish family, Adler studied medicine at the University of Vienna, receiving his degree in 1881. While there, he became a member of Georg von Schönerer's German nationalist organization, a movement he left when its anti-Semitic character became more pronounced. After meeting the German socialists Friedrich Engels (Engels, Friedrich) and August Bebel while traveling in Germany, Switzerland, and England (1883), Adler became a dedicated socialist. He remained a lifelong friend of and correspondent with Engels.

Adler founded and headed the socialist weekly Gleichheit (1886–89, “Equality”) and, after its ban, published the Arbeiter Zeitung (“Workers' Paper”), which became the socialists' main organ. He was chiefly responsible for founding the united Social Democratic Party of Austria (December 1888–January 1889), in which he remained a leading figure, and he made it into a multinational party advocating federalism and autonomy for the peoples of Austria-Hungary. He was elected to the Lower Austrian Landtag (Diet) in 1905 and quickly became a leader in the fight for universal suffrage (introduced in 1907).

The breakaway of the Czech Social Democrats in 1911 frustrated Adler's efforts to maintain a single, united party. During World War I, he worked for a peaceful settlement at the abortive socialist conference in Stockholm (1917). Entering the new Austrian government in October 1918, he advocated the Anschluss (unification) of the rump Austrian state with Germany but died before he could pursue this project.