Jacob Marschak (Russian: Яков Израилевич (Срулевич) Маршак; 22 July 1898 – 27 July 1977) was a Russian and American economist. Affiliated with the Mensheviks in his youth, he had a brief participation in the Russian Revolution, serving as labour minister in the Terek Soviet Republic before emigrating. In exile he developed a successful scholarly career, first in Germany and later achieving prominence in United States academia.

Born in a Jewish family of Kiev, Jacob Marschak (until 1933 Jakob) was the son of a jeweler. During his studies, he joined the social democratic Menshevik Party, becoming a member of the Menshevik International Caucus. In 1918, he was the labor minister in the Terek Soviet Republic. In 1919, he emigrated to Germany, where he studied at the University of Berlin and the University of Heidelberg.

Early in his émigré period he remained active in Russian socialist circles, contributing to the Menshevik journal Sotsialisticheskii vestnik, published in Berlin. From 1922 to 1926, he was a journalist, and in 1928, he joined the new Kiel Institut für Weltwirtschaft. With the gathering Nazi storm, he emigrated to England, where he went to Oxford to teach at the Oxford Institute of Statistics, which was funded by the Rockefeller Foundation, allowing him to emigrate to the United States in 1939. After teaching at the New School for Social Research, in 1943, he went to University of Chicago, where he led the Cowles Commission. He followed the commission's move to Yale University, and he then became a professor at UCLA in 1960.

In 1972, he co-founded Team Theory with Roy Radner.

Marschak was fluent in approximately one dozen languages. Shortly before he was due to become president of the American Economic Association, he died from a cardiac arrest.

UCLA sponsors the recurring Jacob Marschak Interdisciplinary Colloquium on Mathematics in the Behavior Sciences.