William James Pirrie, 1st Viscount Pirrie, KP, PC, PC (Ire) (31 May 1847 – 7 June 1924) was a leading British shipbuilder and businessman. He was chairman of Harland and Wolff, shipbuilders, between 1895 and 1924, and also served as Lord Mayor of Belfast between 1896 and 1898. He was ennobled as Baron Pirrie in 1906, appointed a Knight of the Order of St Patrick in 1908 and made Viscount Pirrie in 1921. In the months leading up to the 1912 disaster, Lord Pirrie was questioned about the number of life boats aboard the Olympic-class ships. He responded that the great ships were unsinkable and the rafts were to save others. This would haunt him forever. In Belfast he was, on other grounds, already a controversial figure: a Protestant employer associated as a leading Liberal with a policy of Home Rule for Ireland.
Pirrie was born in Quebec City, Canada East, the son of James Alexander Pirrie and Eliza Swan (Montgomery) Pirrie, who were both Irish. He was taken back to Ireland when he was two years old and spent his childhood at Conlig, County Down. Belonging to a prominent family, his nephews included J. M. Andrews, who would later become Prime Minister of Northern Ireland, Thomas Andrews, builder of the , and Sir James Andrews, 1st Baronet, the Lord Chief Justice of Northern Ireland.
Pirrie was educated at the Royal Belfast Academical Institution before entering Harland and Wolff shipyard as a gentleman apprentice in 1862. Twelve years later he was made a partner in the firm, and on the death of Sir Edward Harland in 1895 he became its chairman, a position he was to hold until his death. As well as overseeing the world's largest shipyard, Pirrie was elected Lord Mayor of Belfast in 1896, and was re-elected to the office as well as made an Irish Privy Counsellor the following year. He became Belfast's first honorary freeman in 1898, and served in the same year as High Sheriff of Antrim and subsequently of County Down. In February 1900 he was elected President of the Chamber of Shipping of the United Kingdom, where he had been vice-president the previous year.