William Rosenau (1865, Wollstein, Province of Posen, Prussia - 1943, United States) was a leader of Reform Judaism in the beginning of the twentieth century in the United States.
In 1876, Rosenau immigrated to the United States. He received a B.A. from the University of Cincinnati and a Ph.D from Johns Hopkins University. He also got rabbinic ordination from Hebrew Union College.
He served as rabbi in Omaha, Nebraska and in Congregation Oheb Shalom of Baltimore, Maryland. He was also on the facaulty of Johns Hopkins University.
Rosenau was considered a "radical reformer" regarding ritual and he was a member of the anti-Zionist American Council for Judaism.
He was an editor of the revised editions of the Union Prayer Book and the Union Haggadah.