Deyika Nzeribe, 50, was a former chair of the party's Manchester branch and had stood at several local elections.
His campaign manager Astrid Johnson said party members were in "total shock" after losing a great friend and colleague.
The father-of-three and long-time green community activist, from Hulme, launched his campaign in October.
The Green Party's candidate for May's mayoral elections for Greater Manchester has died of a heart attack.