Vainius or Voin (died between 1338 and 1342) was the Lithuanian Prince of Polotsk from 1315 to his death.[1] Very little is known about Vainius, brother of Gediminas, Grand Duke of Lithuania.[1] He is mentioned in written sources in 1324 for the first time. In 1328 he, already as Prince of Polotsk, signed a treaty with the Livonian Order and Novgorod.[1] His only known son Liubartas died in 1342 during fights with the Livonian Order.[1]