John Gast (1772–1837) was an English shipwright and labour activist, an early trade unionist.
He worked in the Deptford shipyards in south-east London; he was also associated with neighbouring Rotherhithe, where he lived for a time at 14 Lucas Street.
Having unsuccessfully tried to found a labour organisation during the 1790s, Gast helped organise the 'Hearts of Oak Benefit Society' during a shipwrights' strike in 1802 and was advocating workers' rights in radical pamphlets such as Calumny Defeated; or, A Compleat Vindication of the Conduct of the Working Shipwrights, during the late Disputes with their Employers (1802).
Employers were furious and lobbied for the Acts’ restoration, prompting the emergence of workers' movements to resist such steps; Gast founded a pioneering Trades Newspaper as part of this resistance, with Joseph Clinton Robertson.
E. P. Thompson suggested, in The Making of the English Working Class, that in the history of working-class movements between 1780 and 1832, he was one of three, with John Doherty and Gravener Henson, who had been outstanding leaders.