William Duffy, Bill Duffy or Billy Duffy may refer to:

• Bill Duffy (basketball), American basketball player and player agent
• Bill Duffy (sportsman) (1866–1959), Australian sportsman
• Billy Duffy (born 1961), English musician
• Billy Duffy (hurler) (1931–2005), Irish hurler
• William Duffy, a pseudonym of William Dufris (1958–2020), an American voice actor
• William Duffy (bishop), English Roman Catholic bishop
• William Duffy (politician) (1865–1945), Irish nationalist politician
• William James Duffy (1888–1946), Canadian politician
• William J. Duffy (1916–2013), American jurist and legislator