The Workers Party of Britain leader will now stand in the Batley and Spen by-election, set for July 1.
The constituency, which made headlines in June 2016 after the murder of its sitting MP Jo Cox is considered a prime target for the Conservatives after their decisive victory in the former Labour stronghold of Hartlepool in May.
Sir Keir Starmer’s party has selected Ms Cox’s sister, local campaigner Kim Leadbeater, as its candidate.
But despite the selection of the popular local figure - as well as key demographic differences between Batley and Spen and Hartlepool - Labour sources suggest the party is fearful of another rout by the Conservatives.
‘It’s curtains for Keir Starmer’
In a direct appeal to disaffected Labour supporters, Mr Galloway said his candidacy was a direct challenge to Mr Starmer’s leadership of the party.
The former Respect MP said: “On July 1, I'm standing against Keir Starmer. If Keir Starmer loses this by election, it's curtains for Keir Starmer.
“So if, for whatever reason, you think that the current leader of the Labour Party needs to be replaced, I'm your man.”
Mr Galloway will also face off against local Councillor Ryan Stephenson of the Conservatives, Jo Conchie of the Lib Dems and Corey Robinson of the Yorkshire Party.