Sir William Arrol (13 February 1839 – 20 February 1913) was a Scottish civil engineer, bridge builder, and Liberal Unionist Party politician. The son of a spinner, Arrol was born in Houston, Renfrewshire, and started work in a cotton mill at only 9 years of age, prior to commencing training as a blacksmith by age 13, and going on to learn mechanics and hydraulics at night school.
In 1863 he joined a company of bridge manufacturers in Glasgow, but by 1872 he had established his own business, the Dalmarnock Iron Works, in the east end of the city. The business evolved to become Sir William Arrol & Co., a large international civil engineering business.
Projects undertaken by the business under his leadership included the replacement for the Tay Bridge (completed in 1887), the Forth Bridge (completed in 1890) and Tower Bridge (completed in 1894). He was also contracted by the Harland and Wolff Shipyard, Belfast, to construct a large gantry (known as the Arrol Gantry) for the construction of three new super-liners, one of which was the RMS Titanic.