The 2021 Millers Oils Ginetta GT4 Supercup is a multi-event, one make GT motor racing championship held across England and Scotland. The championship features a mix of professional motor racing teams and privately funded drivers, competing in Ginetta G55s that conform to the technical regulations for the championship. It forms part of the extensive program of support categories built up around the BTCC centrepiece. It is the eleventh Ginetta GT4 Supercup, having rebranded from the Ginetta G50 Cup, which ran between 2008 and 2010. The season commenced on 12 June at Snetterton and concludes on 24 October at Brands Hatch, utilising the GP circuit, after twenty-three races held at eight meetings, all in support of the 2021 British Touring Car Championship.
For 2021 the amateur class was removed, seeing a return to a two class structure.