R. Dr. Isaac Breuer (1883–1946) was a Jewish rabbi in the German Neo-Orthodoxy movement of his maternal grandfather Samson Raphael Hirsch, and was the first president of Po'alei Agudat Israel. Dr. Breuer was born in Papa, Hungary to Salomon Breuer, and lived most of his years in Frankfurt. He attended Hirsch's Realschule school, and received rabbinical ordination at age 20 from his own father's yeshiva. He studied law, jurisprudence, and philosophy at Marburg University, and until 1936 practiced law in Frankfort.

In 1936 Dr. Breuer moved to Jerusalem, organized and became first president of Po'alei Agudat Israel, and represented the Agudah before the Peel and Anglo American Commissions. He believed that Agudism could help prepare Jews to live in their ancestral home. Dr. Breuer died in Jerusalem in 1946.

Among his published works are: Judenproblem (1918), a polemic against the perceived opponents of Orthodoxy, Zionists and Reform Jews; Messiasspuren (1918); Concepts of Judaism Levinger, Jacob S. (ed.); Jerusalem: eine historische Erzählung, a metaphor in novella form of what negative consequences might follow from the agenda of Zionists.