Yahya Jammeh, who took power in a coup in 1994, cited “abnormalities” in the vote and called for fresh elections.
He was defeated by Adama Barrow, who won more than 43% of this month’s vote.
Adama Barrow accused Yahya Jammeh of damaging democracy by refusing to accept the result. His transition team said the president-elect was safe.
The results were revised by the country’s electoral commission on December 5, when it emerged that the ballots for one area were added incorrectly, swelling Adama Barrow’s vote.
The error, which also added votes to the other candidates, “has not changed the status quo” of the result, the commission said.
However, it narrowed Adama Barrow’s margin of victory from 9% to 4%.
Yahya Jammeh said that he now rejected the results of the election “in totality”.
“After a thorough investigation, I have decided to reject the outcome of the recent election,” he said.
“I lament serious and unacceptable abnormalities which have reportedly transpired during the electoral process.
“I recommend fresh and transparent elections which will be officiated by a God-fearing and independent electoral commission.”
Adama Barrow’s spokesperson said the head of the army, General Ousman Badjie, supported the president-elect, having pledged his allegiance after the initial result.
The US has “strongly condemned” Yahya Jammeh’s rejection of the result.
State Department spokesman Mark Toner said: “This action is a reprehensible and unacceptable breach of faith with the people of The Gambia and an egregious attempt to undermine a credible election process and remain in power illegitimately.”
The streets of the capital, Banjul, were reported to be calm on December 9 although soldiers were seen placing sandbags in strategic locations across the city, AFP news agency reports.
Only last week, Yahya Jammeh was shown on state TV calling Adama Barrow to wish him well.
“You are the elected president of The Gambia, and I wish you all the best. I have no ill will,” he said at the time.
Adama Barrow, a property developer, is due to take office in late January.
The Gambia is the smallest country on mainland Africa, with a population of fewer than two million.
In his 22 years in power, Yahya Jammeh acquired a reputation as a ruthless leader.
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