In early 2016, a newly arrived priest at the cathedral started to suspect the sexton of pilfering after noticing a drop in collection money.
The priest alerted the police, who put a trap in the box which would spray red paint on anyone trying to break in.
The sexton confessed after being caught with red marks on his hands.
The French resident of Cap d'Ail admitted that he had cut a second copy of the box's key before the priest had relieved him of the job of sorting the collection money.
In court this week, the defendant said he stole the money – an estimated €3,000 ($3500) – from the poor box to pay for his son's wedding.
The Monaco court gave the man a five-month suspended prison sentence and a €1,000 fine.
Saint Nicholas Cathedral collected €136,000 in 2016 and €145,000 in 2015, according to information gathered during the investigation.