|Yahya Jammeh (Right) and Adama Barrow (Left)|
In a reversal of his acceptance of the result last week, Mr Jammeh cited "abnormalities" in the vote and called for fresh elections.
Mr Jammeh, who came to power in a coup in 1994, suffered a surprise defeat to Adama Barrow, who won more than 45% of the vote.
Speaking on state TV, he said he now rejected the results "in totality".
"After a thorough investigation, I have decided to reject the outcome of the recent election," Mr Jammeh 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."
According to the electoral commission, Mr Barrow won 263,515 votes (45.5%) in the election on 1 December, while President Jammeh took 212,099 (36.7%). A third-party candidate, Mama Kandeh, won 102,969 (17.8%).
Mr Barrow, a property developer, is due to take office in late January. After his win, Mr Barrow hailed a "new Gambia".
The Gambia is the smallest country on mainland Africa, with a population of fewer than two million. Tourism has become its fastest-growing sector.