An udarnik (Russian: уда́рник, IPA: [ʊˈdarnʲɪk]; English plural udarniks or udarniki), also known in English as a shock worker or strike worker (collectively known as shock brigades or a shock labour team) was a highly productive worker in the Soviet Union, the Eastern Bloc, and other communist countries. The term derived from the expression "udarny trud" for "superproductive, enthusiastic labour".

In the Soviet Union, the term was linked to Shock worker of Communist Labour (Ударник коммунистического труда), a Soviet honorary title, as well as Alexey Stakhanov and the movement named after him. However, the terminology of shock workers has also been used in other socialist states, most notably in the People's Republic of China, North Korea, the People's Republic of Bulgaria, and the Socialist Federal Republic of Yugoslavia.

Soviet shock workers were not always necessarily citizens of the USSR, as one British communist and trade union leader Jessie Eden, was elected one at the Stalin automotive plant (later renamed the ZiL automotives).

The ideology behind promoting shock labour was that through socialist emulation the rest of the workforce would learn from the vanguard.