Mayor Steve Arnold said Walt Anderson tendered his retirement papers following an in camera discussion this month.
"Chief Anderson has been with us for a number of years, and is at retirement age now and he's decided to take a retirement, and so council and senior staff will have to post for a new fire chief," said Arnold. "His retirement's effective at the end of September."
Mayor Arnold would not confirm to Blackburn News that the chief's decision was in response to a controversial post made on social media in late summer.
"From council's perspective, we went all through the process, did the investigation and had the interview with Chief Anderson, and it was within a few days after we had those discussions he decided it was time to retire."
The social media post was flagged for inciting violence.
Arnold said the investigation was discussed by council in closed session and the results would not be released.
Anderson was hired as the township's director of emergency services, which includes the role of fire chief, in 2013.
The position oversees 180 volunteer firefighters and two full-time deputy chiefs.
St. Clair Township has six fire stations and its emergency services headquarters is shared with Lambton OPP in a building on Lyndoch Street in Corunna.