The Timok (Serbian and Bulgarian Cyrillic: Тимок, Romanian: Timoc) is a river in eastern Serbia and for the last 15 km of its run a border between eastern Serbia and western Bulgaria.

It derives the names in all these from the name it had in antiquity, Latin: Timacus.

It is a very branchy system of many shorter rivers, a large number of them having the same name (Timok), only clarified with adjectives. From the farthest source in the system, that of the Svrljiški Timok, until its confluence into the Danube (as Veliki Timok), the Timok is 203 km long. The river flows through Serbia and for the last 15 km forms the border between Serbia and Bulgaria. The area of the river basin is 4,630 km². The Timok Valley is known for the most important Romanian-speaking population in Eastern Serbia.

Timok Cove in Rugged Island in the South Shetland Islands, Antarctica is named after Timok River.

The Timok, also named Veliki Timok to distinguish it from its tributaries, is formed by the confluence of the rivers Beli Timok ("White Timok") and Crni Timok ("Black Timok") at Zaječar. The Beli Timok is formed by the confluence of the rivers Svrljiški Timok ("Svrljig Timok") and Trgoviški Timok ("Trgovište Timok") at Knjaževac.