The Tikhvinka (Russian: Тихвинка) is a river in Boksitogorsky and Tikhvinsky Districts of Leningrad Oblast, Russia, a right and the biggest tributary of the Syas.

The source of the Tikhvinka is in Lake Yeglino in Boksitogorsky District, several kilometers northwest of the urban-type settlement of Yefimovsky.

The river flows northwest, through Lake Ozerskoye, turns west and eventually southwest.

Downstream of Astrachi, the Tikhvinka enters Tikhvinsky District and flows through the town of Tikhvin.

The whole course of the Tikhvinka is a part of the Tikhvinskaya water system, one of the waterways constructed in the early 19th century to connect the basins of the Volga and the Neva.