Justin Hilary O'Byrne, AO (1 June 1912 – 10 November 1993) was an Australian politician who served as a Senator for Tasmania from 1947 to 1981, representing the Australian Labor Party (ALP). He was President of the Senate from 1974 to 1975, including for the duration of the 1975 constitutional crisis that resulted in the dismissal of the Whitlam Government.