Chief of Staff Fritz Longchamp confirmed Preval's exit date of May 14 in a phone interview with The Associated Press following uncertainty about the Haitian leader's plans.
Preval's term had been scheduled to end Monday, but his successor will not be elected until Haiti holds a presidential runoff March 20. He had been silent about his intentions in recent days, leading to rumors that he might appoint a temporary successor.
"He will stay in office until May 14. He will not leave today," Longchamp said.
An emergency law passed by members of Preval's former party in an expiring Senate allows him to remain in office for up to three more months because his 2006 inauguration was delayed.
The U.S. and other nations had signaled they agreed with Preval staying in office for a few months past the end of his term to avoid a power vacuum in Haiti, where foreign governments have collectively spent billions on recovery efforts after last year's devastating earthquake - and pledged billions more for reconstruction.
"The United States believes that a peaceful and orderly transition between President Preval and his elected successor is important for Haiti. The president and the people of Haiti will have to decide on the framework for this transition," said Jon Piechowski, spokesman for the U.S. Embassy in Port-au-Prince.
Mark Schneider, special adviser on Latin America for the International Crisis Group, said there will likely be "greater stability and more movement on reconstruction with this situation ... than if (Preval) were to name a temporary successor - which would clearly be unconstitutional."
Throughout most of the day, about 50 anti-Preval demonstrators protested outside the quake-destroyed National Palace, blocking traffic with overturned trash bins and burning tires. Hundreds of onlookers watched as protesters hurled rocks and chanted "Preval is a crook!"
"Preval has to go today. Today, Preval is unconstitutional," one man shouted repeatedly along the debris-littered streets.
Haitian police chased after the stone-throwing demonstrators. Patrolmen trained their guns down side streets and dismantled barricades. Heavily armed U.N. peacekeepers massed several blocks away, but the small band of protesters was later dispersed by police.
Elsewhere in Port-au-Prince, markets were bustling with shoppers and the intricately painted group taxis known as "tap-taps" ferried Haitians around. But most schools remained closed, fearing violence.
Preval is deeply unpopular, especially in urban areas, after years of continued poverty and following his perceived inaction in response to the earthquake. Many of those who gawked at the demonstrators agreed they wanted to see the end of Preval's term.
"I am not in the position of my brothers here, but it is very hard for the people. Misery is killing people, so we need a change," said Demis Mesidor, an auto parts trader whose home was destroyed in the quake.
Last week, Haiti's provisional electoral council decided to eliminate Preval's government-backed candidate, Jude Celestin, from the presidential runoff. The decision ended a standoff with the country's international partners who questioned an earlier official count showing Celestin had qualified for the runoff.
Instead, Mirlande Manigat, who was the top vote-getter among 11 candidates on the initial ballot Nov. 28, will face popular singer Michel "Sweet Micky" Martelly, the No. 2 finisher.
Campaigning for the second round, originally slated for January, is set to begin Feb. 17. The final count - the naming of Haiti's next president - is not foreseen until April 16.
In a Monday statement, the Boston-based Institute for Justice and Democracy in Haiti, which is pro-Jean-Bertrand Aristide, a former president who was ousted in 2004 in a violent rebellion, argued Haiti's electoral council did not validly approve the runoff election.
The group said one member reportedly claims only four of the panel's eight members approved the decision while the panel's bylaws require that decisions be made by an "absolute majority of its members."
Another vocal critic is the Center for Economic and Policy Research, a left-leaning Washington think tank that maintains the entire election should be thrown out. The center said Monday that it makes sense for Preval to stay on for three months - but that the decision about the second round of elections was profoundly flawed.
"It is not clear how a legitimate government will be formed from this election," said co-director Mark Weisbrot.
Associated Press writer Jacob Kushner in Port-au-Prince contributed to this report.