Mr Galloway, who was expelled from Labour in 2003, said he is standing as a candidate for his Workers Party of Britain in the poll on July 1.
In a video posted on his social media feeds, filmed at Batley bus station, he said: “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 is aiming to provide a further headache for Labour which will be looking to secure every vote it can get to avoid a similar fate to the party’s dramatic defeat by the Tories in Hartlepool earlier this month.
Kim Leadbeater – the sister of Jo Cox, who was Batley and Spen’s MP until she was murdered in the constituency in 2016 – was selected as Labour’s candidate earlier this week.
But the Conservatives, who have chosen Leeds councillor Ryan Stephenson as their candidate, will also be looking closely at who else is on the ballot paper, potentially complicating the electoral maths.
In the December 2019 general election, Paul Halloran, representing the Heavy Woollen District Independents, came third in Batley and Spen, securing more than 6,000 votes – 12% of the total.
Mr Halloran has not yet confirmed whether he is standing on July 1.
And the Yorkshire Party, which has selected local engineer Corey Robinson to fight the by-election, came third in the recent West Yorkshire mayoral election, with nearly 10% of the vote, beating the Greens and the Liberal Democrats.
It was the West Yorkshire mayoral poll which prompted the Batley and Spen by-election as the winner, Labour’s Tracy Brabin, had to relinquish her parliamentary seat due to her new role.
Ms Brabin was elected in Batley and Spen in 2016 in a largely uncontested by-election following Miss Cox’s murder.
In the 2019 general election she won for Labour with a majority of 3,525 over the Conservatives.
Campaigning in Batley and Spen could be complicated by coronavirus, with Kirklees one of the areas where people have been encouraged to “minimise travel” due to the spread of the Indian variant.
Mr Galloway has had a colourful political career, including a spell as MP for another West Yorkshire seat – Bradford West – between 2012 and 2015.