Will Patterson, 33, chairman of the Wigan and Leigh Green Party, is to run following the death of Deyika Nzeribe.
"These are tough circumstances to run in but I'm honoured that I have the chance to continue the great work Deyika started," he said.
All the main parties have chosen their candidates for the 4 May election.
Green campaigner and father-of-three Mr Nzeribe from Hulme was 50 years old when he died  in January.
His campaign manager Astrid Johnson said party members were in "total shock" after losing a great friend and colleague.
He was a former chair of the party's Manchester branch and had stood at several local elections.
The Green Party has announced its contender for the Greater Manchester mayoral elections after its first candidate died from a heart attack.