Sir Jim, who is based in Monaco, was a prominent Brexiteer but has criticised the Tories’ handling of the UK’s exit from the EU.
The endorsement of Sir Keir marks a change in attitude for the entrepreneur, who has often criticised Labour in the past.
He previously claimed that his move to Monaco in 2018 was fuelled by fears that former leader Jeremy Corbyn would win the 2019 general election.
Monaco is considered a tax haven as residents do not pay income tax or capital gains taxes.
Sir Jim also previously admitted that Ineos shifted its tax base to Switzerland in 2010 after Gordon Brown’s government refused to help the company defer a large tax bill.
He said the move left him “p—– off”, although he has since made the UK his company’s tax domicile once again.
Sir Keir and the Labour Party have spent months courting the business world in a bid to win support.
‘Very foolish’ to abolish non-dom status
Sir Jim’s endorsement is an apparent sign that the party has convinced City grandees that they can be trusted to steer the British economy.
The billionaire chemicals mogul, who is worth £11.9bn, also criticised Jeremy Hunt’s plan to abolish the non-dom status for foreign nationals working in the UK, saying it was “very foolish”.
He said: “You’ve got 60,000 very wealthy people in London, why would you want to encourage them to leave? It doesn’t make any sense to me really because they all bring enormous value.”
Sir Jim founded Ineos in 1998 after working in the oil industry and at the private equity firm Advent International.
He bought a 25pc stake in Manchester United through Ineos last December for about £1bn.
As part of the deal, Ineos was permitted to take control of the club’s football operations.
Ineos has a significant involvement with sport, including sponsoring the British cycling team, formerly known as Team Sky.
Sir Jim was knighted in 2018 under Theresa May’s government for services to business.