The care home committed an "error in judgment" in allowing the visit, the municipality of Mol in the Flanders region said in a statement Saturday.
"In-depth scientific research" would be needed to definitively say whether the visit was the cause of an outbreak at the Hemelrijck care home, the municipality said, adding that 61 residents and 14 staff members have tested positive so far.
One resident who was already receiving palliative care has died, and another resident with severe symptoms is being given oxygen therapy inside the care home, the municipality said. The "vast majority" of those infected are doing well and not showing symptoms, it added.
On December 6 each year, Belgians wake up and rush to see if St. Nicholas, or Sinterklaas, as he is known in Flemish-speaking parts of the country, has left presents for them.
The man who played Sinterklaas, who is the son of a resident, tested positive for coronavirus after his visit.
He "was not feeling sick at the time of the visit" and "the activity was not cleared beforehand with the crisis center, otherwise negative advice would have been given," the municipality said.
CNN has reached out to the care home's operator, Armonea, for comment.
"Contrary to reports in the media, St. Nicholas did not visit every room. The management reassures us that the saint only visited common areas, including the seating areas," the municipality said.
"The saint maintained distance at all times from the residents, and didn't remain in any area longer than a few minutes. The saint did not hand out presents."
Belgium has been hit hard by the pandemic. The nation of 11.5 million has recorded 608,137 coronavirus cases and 17.951 deaths, according to data from Johns Hopkins University.
At the end of October, pressure on the health service was so great that health workers in some hospitals in Liège, Belgium's third largest city, were asked to continue working even if they tested positive for Covid-19 -- as long as they were not showing any symptoms of the disease.