Vyachko (Latvian: Vetseka) was a Russian prince who fought against the expansionism of the Livonian Knights at the turn of the 13th century. His name is the Old Novgorod dialect form of Vyacheslav and his father is supposed to have been a Rurikid Prince of Drutsk.
When the Knights first arrived to present-day Latvia, Vyachko ruled the fortress of Kukeinos. Although his principality is believed to have been subject to Polotsk, senior princes did nothing to help him withstand the Knights' pressure. During one of the raids he was captured by Albert of Buxhoeveden and delivered in chains to Riga, where the local archbishop set him free. Thereupon Vyachko burnt his capital and the fortress of Cesis (Кесь) he had founded and retreated to Rus.
In 1223, the Novgorod Republic sent Vyachko to defend the fortress of Yuriev (Dorpat) against the Knights. Although his druzhina was small, Vyachko managed to take the city, to impose a tribute on the land, and to launch several raids against the Knights. In response, Albert besieged Yuriev with a large force and offered a peace settlement. However, Vyachko refused to surrender and chose to die with all of this supporters during the storm of the fortress.