Tosh McDonald, executive committee president of the Associated Society of Locomotive Engineers and Firemen (ASLEF), was chosen as the party candidate by Labour members in the Town ward.
Coun John McHale is standing down and a by-election has been called for Thursday, June 14. It's understood Mr McHale is standing down due to health reasons.
Mr McHale was re-elected last year with over 1,800 votes along with fellow party colleagues Dave Shaw and Nikki McDonald.
The Yorkshire Party has announced deputy leader Chris Whitwood will be their candidate. The full list of candidates hoping to take the vacant seat in the council chamber will be released at 4pm on Thursday, May 17.
Rail union chief McDonald, who hails from Doncaster, is the favourite to be elected and is a key friend and ally of Labour leader Jeremy Corbyn. He was congratulated by Town ward councillor Dave Shaw.
He tweeted: "Congratulations to @McDonaldTosh for his selection to represent #Labour in #Doncaster Town Ward in the upcoming by-election."
Mr McDonald was re-elected as Aslef president in January unopposed - his fourth stint in the role. Her confirmed he will be standing down at the end of the year.
But the union boss said he wasn't taking anything for granted and paid tribute to the outgoing councillor.
"I'd like to thank John for his service. He's been a fantastic councillor and representative for our ward for many years," Mr McDonald said.
"I've had a message from him in Ireland and it's great to have his support. If I am elected, I hope I can show the same energy, passion and dedication he has shown."
Dame Rosie Winterton, MP for Doncaster Central, also paid tribute to former councillor McHale.
She said: “John McHale has been a brilliant local councillor, who has represented his constituents with true dedication and commitment.
"I know he is very much respected amongst the community as many local people have told me over the years of how appreciative they are of the work he has done on their behalf.
"John will be sorely missed from the council but I wish him well in his future endeavours.”
Residents of Town ward will be sent a polling card in the coming weeks. Anyone who isn't registered to vote will have to do so by Tuesday, May 29.