William James Pirrie, Viscount Pirrie, KP (May 31, 1847) - (June 6, 1924) was a leading Irish shipbuilder and businessman. Born in Quebec, he was educated at the Belfast Royal Academical Institution before entering Harland and Wolff shipyard as an apprentice gentleman 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 a Irish Privy Counsellor the following year. He became Belfast's first honorary freeman in 1898, and also served as High Sheriff of successively County Antrim and County Down. He helped finance the Liberals in Ulster in the 1906 General Election, and that same year, at the height of Harland and Wolff's success, he was created Baron Pirrie, of the City of Belfast. The following year he was appointed Comptroller of the Household of the Lord Lieutenant of Ireland, and in 1908 he was made a Knight of St Patrick.

Pro-Chancellor of the Queen's University of Belfast from 1908 to 1914, Lord Pirrie was also in the years before the First World War a member of the Committee on Irish Finance as well as Lieutenant for the City of Belfast (both 1911). During the war he was a member of the War Office Supply Board, and in 1918 became Comptroller-General of Merchant Shipbuilding, organising British production of merchant ships.

In 1921 Pirrie was elected to the Northern Ireland Senate, and that same year was created Viscount Pirrie, of the City of Belfast. He died three years later of pneumonia on a business trip to South America, and was buried in Belfast City Cemetery.