The 2S31 Vena () is a Russian amphibious self-propelled 120 mm mortar system. "2S31" is its GRAU designation.
The 2S31 consists of a 120mm 2A80 rifled gun-mortar mounted on the chassis of the BMP-3 infantry combat vehicle. It was developed by Joint Stock Venture "Plants of Motovilikha" (). The 2S31's turret is equipped with digital automated fire control system, navigation system, and optical/electronic reconnaissance and target acquisition system. Its 2A80 gun-mortar is capable of launching a high-explosive mortar projectile with a maximum range of . The vehicle is protected by a welded aluminium hull to protect against small arms fire and shrapnel. It can also fire armour-piercing projectiles that can penetrate up to of steel plate at ranges of up to . A separate sighting system is used when engaging in direct fire mode.
Reportedly first combat use of Vena was during Second Nagorno-Karabakh War.
In September 2023, Vena appeared in Vesti program during the report about Russian troops fighting near Bakhmut, confirming its use during the Russian invasion of Ukraine.