Irwin Steingut (October 19, 1893 – September 26, 1952) was an American lawyer, businessman and politician. At the time of his death he had served as a member of the New York Assembly longer than anyone in history. Early in his career he teamed with Brooklyn boss John H. McCooey, who turned Brooklyn into a solidly Democratic power base and dominated its politics for a quarter of a century until his death in 1934. Steingut thereafter became the de facto leader of the Brooklyn Democratic Party. Throughout almost all of his legislative career Republicans held a majority in the New York Assembly, and much of that time Steingut was the Minority Leader. In 1935 for the one year the Democrats had the majority, Steingut was Speaker of the Assembly.