Mr Preval had previously been short of the overall majority needed to win outright, but a change in the way blank ballots were counted gave him the 51 percent needed to avoid a run-off.
"Mr Rene Garcia Preval is credited with 51.15 percent of the votes, based on 96 percent of voting stations counted, and is declared the winner," the electoral council said in a statement.
While crowds danced and sang in the capital Port-au-Prince, the second-placed candidate said Mr Preval's victory was a "reward for violence".
The announcement followed five days of protests over the earlier results that gave Mr Preval 48.76 percent of the vote in the February 7 election.
Mr Preval, a former president, had rejected the partial results, decrying what he said was "massive fraud or gross errors" and encouraging his supporters to demonstrate.
Electoral authorities said blank votes had been distributed on a pro-rata basis among the 32 candidates. This took Mr Preval over the 50 percent mark.
The decision followed intense diplomatic talks aimed at averting an explosion of violence in the volatile Caribbean country that descended into chaos two years ago, when Jean Bertrand Aristide resigned as president and fled into exile.
Leslie Manigat, who took a distant second place, accused the government of caving in to pressure from Mr Preval and the international community.
He called the result "a reward for violence" and "an electoral coup d'etat" that had deprived him of the chance of a second round.
"As we did in the 1988 coup against us, we say good luck to the country," said Mr Manigat, 75, who was president for less than five months before he was ousted in 1988.
The previous partial result had given him just 11 percent of the vote.
Peacekeepers
Brazil leads the 9,500-strong international peacekeeping force in Haiti and Brazilian Foreign Minister Celso Amorim said his country had helped "facilitate" the steps taken in Haiti to solve the dispute.
The Organisation of American States also took part in the talks.
Mr Preval supporters applauded peacekeepers patrolling in armed personnel carriers, in sharp contrast with past violence that left nine peacekeepers dead.
The celebrations contrasted with days of protests that left one person dead and paralysed the city amid outrage over the discovery of ballots at a garbage dump.
The provisional result included more than 85,000 blank votes, which some analysts said appeared unusually high in a country where many voters walked and then waited for hours to cast their ballots.
Mr Preval was president from 1996 to 2001. He served as prime minister in Mr Aristide's government in 1991, but his aides say the two men are no longer in contact.
Like Mr Aristide, Mr Preval is considered a champion of the poor, who make up 77 percent of Haiti's 8.5 million population.
He has pledged to tackle poverty, seek national dialogue, strengthen Haiti's hesitant democracy and boost the country's struggling police force.
"Now that Preval is president we hope we'll have security," said Prophete Nicodeme, a student who got up before dawn to celebrate with neighbours.
"We hope for political and social stability, and less unemployment," he said.
The international community has hailed the February 7 election, despite its chaotic start after four postponements since November, and indicated aid to Haiti would be stepped up to help the struggling country on the track to democracy and stability.
Mr Preval has said he would boost Haiti's 5,000-strong police force and acknowledged he would want the peacekeeping force known as MINUSTAH to remain until stability is established.
The UN Security Council on Tuesday unanimously voted to renew MINUSTAH's mandate for at least another six months.