Sir William Fairbairn, 1st Baronet (of Ardwick) (19 February 1789 – 18 August 1874) was a Scottish structural engineer. Born in Kelso to a local farmer, Fairbairn showed an early mechanical aptitude and served as an apprentice millwright in Newcastle upon Tyne where he befriended the young George Stephenson. He moved to Manchester in 1813 to work for Adam Parkinson and Thomas Hewes. In 1817, he launched his mill-machinery business with James Lillie as Fairburn and Lillie Engine Makers.
Fairbairn was a life-long learner and joined the Institution of Civil Engineers in 1830. In the 1820s and 30s, he and Eaton Hodgkinson conducted a search for an optimal cross section for iron-beams. They designed, for example, the bridge over Water Street for the Liverpool and Manchester Railway, which opened in 1830. In the 1840s, when Robert Stephenson, the son of his youthful friend George, was trying to develop a way of crossing the Menai Strait, he retained both Fairbairn and Hodgkinson as consultants. It was Fairbairn who conceived of the idea of a rectangular tube or box girder to bridge the large gap between Anglesey and North Wales. He conducted many tests on prototypes in his Millwall shipyard and at the site of the bridge, showing how such a tube should be constructed. The design was first used in a shorter span at Conway, and followed by the much larger Britannia Bridge.