The Battle of Tolvajärvi (['tol.va.jær.vi]; Finnish: Tolvajärven–Ägläjärven taistelu, Russian: Битва при Толваярви) was fought on 12 December 1939 between Finland and the Soviet Union.

On 2 December, the Red Army captured Suojärvi, threatening the railroad to the Karelian Isthmus and the rear of the Mannerheim Line.

Simultaneously, the Finnish First and Second Companies of ErP 112 attacked the southern end of Kotisaari Island, but were forced to retreat to the south by early afternoon.

[3] Pajari's attack across the Tolvajärvi peninsula was also delayed until 9:15, meeting resistance 200 yards west of the tourist hotel.

Yet, by the end of the day the Finnish troops had secured the Ristisalmi Strait and the Soviets continued their retreat towards Metsänvaara.

The Red Army's losses along the Tolvajärvi-Aittojoki road included 4000 dead, 580 prisoners, 59 tanks, and 220 machine guns.

According to Allen Chew, for the Finns, "Proportionally, these were the heaviest losses of the entire war," with 630 killed and 1320 wounded.