Alfred Gadenne, 71, was the conservative mayor of Mouscron and a former lawmaker in the regional parliament.
He acted as caretaker for the cemetery next to his home and was apparently killed after going there to lock up for the night.
Mr Gadenne’s body was found by his wife who went to look for him after he failed to return home.
A teenager who blamed Mr Gadenne for his father’s suicide after he was fired from a town hall job has confessed to slashing the politician’s throat, according to public broadcaster RTBF.
RTBF said the suspect was 18 and told police he was avenging his father, who killed himself two years ago after losing his job as a local council employee.
The public prosecutor’s office confirmed a man was under arrest after giving himself up to police at the scene after dark in Mouscron, an industrial border town of 57,000, close to the French city of Lille.
Belgian Prime Minister Charles Michel said: “I have learned with horror of the brutal death of Alfred Gadenne.
“All my thoughts are with his family and friends.”
Philippe Courard, president of the parliament for Belgium’s French-speaking south, tweeted: “Terrifying. What kind of world are we living in?”