Zainsk (Russian: Заи́нск is a town in the Republic of Tatarstan, Russia, located on the Stepnoy Zay River (Kama's tributary), 287 kilometers (178 mi) east of Kazan.

[6] Zainsk Reservoir, along the Zay River, is located near the town.

Within the framework of administrative divisions, Zainsk serves as the administrative center of Zainsky District, even though it is not a part of it.

[1] As an administrative division, it is, together with the settlement of Karmalka, incorporated separately as the town of republic significance of Zainsk—an administrative unit with the status equal to that of the districts.

[1] As a municipal division, the town of republic significance of Zainsk is incorporated within Zainsky Municipal District as Zainsk Urban Settlement.