In summer 1941 all Finnish submarines were once again readied for combat operations and they sailed to the staging area in the Gulf of Finland.
Vesikko's base of operations was to be Vahterpää island near the town of Loviisa.
When the Continuation War started on 25 June, all submarines were ordered to patrol the eastern part of the Gulf of Finland.
On 3 July 1941 Vesikko sank a Soviet merchant ship named Vyborg east of Gogland island.
The attack was made 700 metres (770 yd) from the target; first one torpedo was launched at 13:25 which hit the stern of the target.
The target stopped but did not appear to be sinking so Vesikko fired another torpedo which failed to explode.
Very soon after the strike, three Soviet patrol boats started to chase Vesikko and tried to destroy it with depth charges and salvage the damaged ship but failed to accomplish either task.
Vyborg sank on 3 July at 14:15.

Soviet historiography later downplayed the sinking of Vyborg, insisting that several submarines and German naval bombers had assaulted the ship simultaneously, and that over twenty torpedoes had been launched against it.
During fall 1941 Vesikko operated from Helsinki and made three patrols to the coast of Estonia.
In 1942, equipped with depth charge rack, she acted as an escort to convoys in the Sea of Åland, and hunted suspected hostile submarines near Helsinki.

In the beginning of June 1944, Vesikko escorted the convoys which were evacuating people from the Karelian Isthmus.
Due to the armistice between Finland and the Soviet Union, Vesikko was ordered to return to port on 19 September 1944.
Vesikko sailed the last time as a combat vessel of the Finnish Navy in December 1944.

During wartime, several officers were commanders of the submarine: Ltn.
Kauko Pekkanen (1939), Capt.
Ltn.
Olavi Aittola (1940 and 1941), Capt.
Ltn.
Antti Leino (1942), Capt.
Ltn.
Pentti Airaksinen (1942), Capt.
Ltn.
Eero Pakkala (1943), Capt.
Ltn.
Olavi Syrjänen (1943), and Capt.
Ltn.
Lauri Parma (1944).
In summer 1941 all Finnish submarines were once again readied for combat operations and they sailed to the staging area in the Gulf of Finland. Vesikko's base of operations was to be Vahterpää island near the town of Loviisa. When the Continuation War started on 25 June, all submarines were ordered to patrol the eastern part of the Gulf of Finland. On 3 July 1941 Vesikko sank a Soviet merchant ship named Vyborg east of Gogland island. The attack was made 700 metres (770 yd) from the target; first one torpedo was launched at 13:25 which hit the stern of the target. The target stopped but did not appear to be sinking so Vesikko fired another torpedo which failed to explode. Very soon after the strike, three Soviet patrol boats started to chase Vesikko and tried to destroy it with depth charges and salvage the damaged ship but failed to accomplish either task. Vyborg sank on 3 July at 14:15.
Soviet historiography later downplayed the sinking of Vyborg, insisting that several submarines and German naval bombers had assaulted the ship simultaneously, and that over twenty torpedoes had been launched against it. During fall 1941 Vesikko operated from Helsinki and made three patrols to the coast of Estonia. In 1942, equipped with depth charge rack, she acted as an escort to convoys in the Sea of Åland, and hunted suspected hostile submarines near Helsinki.
In the beginning of June 1944, Vesikko escorted the convoys which were evacuating people from the Karelian Isthmus. Due to the armistice between Finland and the Soviet Union, Vesikko was ordered to return to port on 19 September 1944. Vesikko sailed the last time as a combat vessel of the Finnish Navy in December 1944.
During wartime, several officers were commanders of the submarine: Ltn. Kauko Pekkanen (1939), Capt. Ltn. Olavi Aittola (1940 and 1941), Capt. Ltn. Antti Leino (1942), Capt. Ltn. Pentti Airaksinen (1942), Capt. Ltn. Eero Pakkala (1943), Capt. Ltn. Olavi Syrjänen (1943), and Capt. Ltn. Lauri Parma (1944).
Finnish submarine Vesikko