Vyachko (Russian: Вячко; Latvian: Vetseka) was a Rus'ian prince who fought against the expansionism of the Germanic 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.
At the time in the beginning of the 13th century, when Germans led by Albert of Buxhoeveden and the crusading Livonian Order began to occupy the shores of the Gulf of Riga, Vyachko ruled the fortress of Kukeinos (modern Koknese, Latvia) some 100 km southeast. Although his principality is believed to have been subject to Polotsk, senior princes did nothing to help him withstand the Knights' pressure. According to other sources, it was indeed in return for protection against Lithuanians and Polotsk, that Vyachko gave half of his land to Albert in 1205. During one of the raids he was captured by Albert of Buxhoeveden and delivered in chains to Riga, where a local archbishop set him free. Thereupon Vyachko burnt his capital and the fortress of Kes' (presentely Cēsis, Latvia) and retreated to Rus. By 1209 Kukeinos had been taken over by the Order and the formal sovereignty of Polotsk was finally revoked in 1215.
In 1223, the Novgorod Republic sent Vyachko to defend the Estonian fortress of Yuriev (modern Tartu, Estonia) against the Knights. Although his druzhina was small, Vyachko managed to install himself in the fortress with support from local Estonians and to launch several raids against the Knights. In response, Albert besieged Yuriev in 1224 with a large force and offered a peace settlement. However, Vyachko refused to surrender, choosing to die with all of his supporters when the Knights stormed the fortress.