The sell-off is part of an efficiency programme launched by Fortum a year ago. The sale price has not been disclosed.
Finnish company Fortum is concentrating its combined heat and power production on larger centres and units.
The production capacity of the Kuusamo plant is 6 MW electricity and 19 MW district heat and its staff will be unaffected by the sale.
Adven is a district heating company with 182 employees in Finland and Estonia.