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.