Rene Preval has been declared president of Haiti, following last week's vote marred by claims of irregularities.
Preval supporters poured into the streets to celebrate his win
He gained 51% of the vote after the authorities reached a last-minute deal changing the way blank ballot papers were allocated between the candidates.
The 7 February vote triggered big rallies by Mr Preval's supporters, who had alleged widespread vote-rigging.
Runner-up Leslie Manigat called the ruling "the imposition of a victor" and said it was "a reward for violence".
Jubilant crowds have poured into the streets of the capital, Port-au-Prince, to celebrate Mr Preval's victory.
UN peacekeepers are stepping up security to prevent any rioting by opponents of the winner, reports say.
The UN Security Council earlier this week renewed the mandate of its 9,500-strong mission in Haiti for at least another six months.
Mr Preval was credited with 51.15% of the votes, based on 96% of voting stations counted, the electoral body chief announced.
"Rene Preval... is declared the winner," Max Mathurin said.
His comments came after officials agreed to redistribute more than 80,000 blank votes - or just over 4% of the total tally - on a pro-rata basis between the candidates, taking Mr Preval over the winning threshold.
The decision followed late-night talks between the electoral council, the interim government and the Organization of American States.
Correspondents say the authorities were keen to avoid further violence over the allegations of electoral fraud.
The agreement follows days of demonstrations, fuelled by the apparent discovery of charred ballot papers at a dump near Port-au-Prince.
Mr Preval had warned of more protests if partial results - which would have required a run-off if confirmed - were published as final.
Rene Preval had urged his supporters to keep up protests
The politician insisted he had won the vote, but partial results had suggested he was just short of the 50% needed to be elected outright.
Haiti's interim government had earlier blocked publication of results until an inquiry into the fraud allegations was complete.
The UN Security Council has urged the Haitian authorities fully to investigate the claims.
Haiti - the poorest country in the Americas - was also choosing a 129-member parliament on 7 February.
Support from poor
The candidate of the small L'Espwa (The Hope) party, Mr Preval was once an ally of ousted former President Jean-Bertrand Aristide and has inherited his following among the poor.
An agronomist who studied in Belgium, Mr Preval was active in the movement to oust military ruler Jean-Claude "Baby Doc" Duvalier during the 1970s.
Mr Preval was prime minister for a brief period in Mr Aristide's first administration in the early 1990s.
He replaced Mr Aristide as president between 1996 and 2001. The political situation began to deteriorate by the end of his term.