The virus, which is fatal to pigs but does not infect humans, has been found in dead wild boar piglets in Vederlax.
On Friday, the Finnish Food Authority established a safety zone in the municipalities of Vederlaakso and Miehikkälä, as well as parts of southern Lappeenranta.
Berry and mushroom picking, hunting and dog walking are no longer allowed in the forest. Forestry work is also suspended.
Last year, Estonia had an outbreak of swine fever that led to the culling of more than 50,000 pigs. Sweden was hit by a swine fever outbreak in Fagersta in 2023.
