The fresh announcement he made on state television on Friday throws the future of the West African country into doubt after the unexpected election results ended Jammeh's 22-year rule.
Jammeh had conceded defeat on state TV last week, prompting wild celebrations over the ending of a government that human-rights groups accused of detaining, torturing and killing opponents.
"After a thorough investigation, I have decided to reject the outcome of the recent election. I lament serious and unacceptable abnormalities which have reportedly transpired during the electoral process," Jammeh said, changing his position on the election results.
"I recommend fresh and transparent elections which will be officiated by a god-fearing and independent electoral commission," he said.