Police said a person was killed in the Artibonite Valley, where rival political party supporters shot at each other as voting proceeded Sunday, The Miami Herald reported Monday.
However, Colin Granderson, head of an international observer mission, said the second round of voting was "much improved" from the first.
"The atmosphere was calm and people seem relaxed," he said.
Musician Michel "Sweet Micky" Martelly faced former first lady Mirlande Manigat in the runoff, conducted after November's fraud-riddled election. The winner will succeed Rene Preval.
Preliminary results are expected March 31 and final results likely won't be announced until April 16, election officials said.
Observers reported some precincts lacked sufficient ballots, had no ink to mark thumbs of those who voted and turned away voters whose names were on incomplete voter rolls.
U.S. Embassy officials said they were monitoring the vote, The Washington Post reported.
"Overall, it was a smoother day than November 28," embassy spokesman Jon Piechowski said. "It wasn't perfect, but it was an improvement based on our observations."