William Willoughby may refer to: William Willoughby, 11th Baron Willoughby de Eresby (1482–1526), father of Katherine Willoughby and the largest landowner in Lincolnshire, England William Willoughby, 1st Baron Willoughby of Parham (c. 1515–1570), English baron and nephew of the above William Willoughby (c.1566-1615), MP for Nottingham William Willoughby, 3rd Baron Willoughby of Parham (1584–1617), English peer William Willoughby, 6th Baron Willoughby of Parham (c. 1616–1673), English landowner and politician William Willoughby, 5th Baron Willoughby de Eresby (c. 1370–1409), English baron William Arnson Willoughby (1844–1908), Ontario physician and political figure William F. Willoughby (1867–1960), author of public administration texts Bill Willoughby (born 1957), American former basketball player