William Patrick d'Alton "Bill" Lacy (born 1930 in Howth, County Dublin, Ireland, died 7 February 2010 in the same place) was an Irish racing driver.
Lacy won the Leinster Trophy in 1954 and two years later made his Formula One debut with the Halford Auto Racing Team at the Irish Grand Prix. Lacy retired from the race but returned for the British Grand Prix two races later - this time with the Irish Racing Cars team. Lacy failed to qualify.
Following his brief Formula One stint, Lacy returned to sportscars. He gave this up in 1963 after getting married and began devoting his time to the burgeoning Howth Yacht Club, of which he was Commodore from 1977 to 1979.