Brexit Party leader Mr Farage confirmed Ms Widdecombe had joined on Wednesday morning, saying she would be the lead candidate for the South West.
“This is great news,” he tweeted.
And Ms Widdecombe told the Daily Express: “The public needs to send a very clear message and that is we expect the vote to be respected so just get on with the job of getting us out of the EU.
"If I am elected in Brussels my message to [Jean-Claude] Juncker and company will be very simple, very loud and very clear. Nous allons [we go].”
Ms Widdecombe, 71, was the Conservative MP for Maidstone – and latterly Maidstone and The Weald – between 1987 and 2010 until she retired from politics.
In the intervening years, she became a media personality, even appearing on Celebrity Big Brother last year.
But nine years later, the hardline Eurosceptic has returned to frontline politics and joined other Tory defectors in the Brexit Party.
They include Annunziata Rees-Mogg, sister of Tory MP Jacob, and Richard Tice, who last year was on the list of contenders to become Conservative candidate for mayor of London.