Lengvenis (Lengewin, Langwinus) was one of the local dukes in the early Grand Duchy of Lithuania during the reign of King Mindaugas (1230s–1263). First mentioned in 1242, Lengvenis was a nephew of Mindaugas (a son of his sister). He was mentioned for the last time in June 1260 as a witness to Mindaugas' act granting Lithuania to the Livonian Order in case he died without a legitimate heir. However, the act is considered to be a Livonian forgery.