Jammeh received 67 per cent of the vote, compared with 27 per cent for Ousainou Darboe, the main opposition candidate, said Joseph Colley, head of communications for the electoral commission after ballot-counting ended on Saturday.
The results were also read out on state radio and television, and welcomed at a gigantic beach victory party attended by thousands of Jammeh supporters wearing his green campaign colors.
"My vision is to make the Gambia one of the best countries on earth and not only in Africa," Jammeh told supporters after the win was confirmed.
Voter turnout was about 59 per cent, much lower than was expected, Colley said.
Approximately 87 per cent of the population turned out to vote in Gambia's last presidential election in 2001, he said.
Close to 670,000 of Gambia's 1.6 million people are registered to vote.
The victory was widely expected following the adoption of a constitutional amendment that awarded the presidency to the candidate with the largest share of votes - no matter how slim the margin. Gambia has no term limits for the presidency.
Jammeh seized power in a 1994 coup and has ruled Gambia for 12 years, winning elections in 1996 and 2001 that opposition groups said were rigged. International observers disputed the 1996 vote, but cleared the 2001 election as free and fair.
Gambians braved heavy rain and street flooding to vote Friday, holding brightly colored umbrellas as they stood in line outside stadiums and schools to drop a marble in one of the bins marked for the three candidates.
An observer team sent by Britain and its former colonies said the vote was well-organized, but that an "abuse of incumbency" before the balloting may have prevented the election from being entirely fair.
"We have been made aware of the events in the lead up to the election day that may have impacted the outcome," said Salim Ahmed Salim, Tanzania's former prime minister and head of the 15-person team.
In a statement, Salim pointed to an allegedly apolitical presidential tour that occurred during the campaign period and the open demonstration of support by security services officials as examples.