William Patrick Callahan (born June 17, 1950) is an American prelate of the Roman Catholic Church who has served as bishop of the Diocese of La Crosse in Wisconsin since 2010. Callahan previously served as an auxiliary bishop of the Archdiocese of Milwaukee in Wisconsin. After the archbishop of Milwaukee, Timothy Dolan, was named Archbishop of New York, Callahan was elected the apostolic administrator of the Archdiocese of Milwaukee, serving until November 14, 2009.