William John Duffy (7 April 1865 – 1 January 1945) was an Irish nationalist politician from County Galway. He was one of the few people to have served both in the United Kingdom House of Commons and in Dáil Éireann.
Duffy was elected at the 1900 general election as Member of Parliament (MP) for South Galway, as an Irish Parliamentary Party candidate, and held the seat until his defeat at the 1918 general election by the Sinn Féin candidate, Frank Fahy.
In the June 1927 Irish general election, he was elected as National League Party Teachta Dála (TD) for Galway. He did not contest the September 1927 Irish general election.