William Arrol (1839 – 1913) was a Scottish civil engineer and bridge builder.

The son of a spinner, he was born in Houston, Renfrewshire, and started work in a cotton mill at only 9 years of age. He started training as a blacksmith by age 13, and went on to learn mechanics and hydraulics at night school. In 1863 he joined a company of bridge manufacturers in Glasgow, but by 1872 had established his own business, the Dalmarnock Iron Works.

In 1878, he secured the contract for the Caledonian Railway Bridge over the Clyde, and In 1882 he was awarded the reconstruction contract for the Tay Rail Bridge, which had collapsed in 1879. His company went on to construct the magnificent Forth Bridge which was completed in 1890. At the time, the Tay and Forth bridges were the largest of their type in the world. Other notable bridges followed, including: Tower Bridge in London, completed in 1894, the Nile Bridge in Egypt and the Hawkesbury Bridge in Australia. He also constructed Bankside Power Station in London, now the Tate Modern Art Gallery.

Arrol was knighted in 1890, and elected as the Liberal Member of Parliament (MP) for South Ayrshire at the 1895 general election, serving the constituency until 1906.

His company was contracted by 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 called 'Titanic'.

He spent the latter years of his life on his estate at Seafield, near Ayr, where he died in 1913.