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A closed city or closed town is a settlement where travel or residency restrictions are applied so that specific authorization is required to visit or remain overnight.

They may be sensitive military establishments or secret research installations which require much more space or freedom than is available in a conventional military base. There may also be a wider variety of permanent residents including close family members of workers or trusted traders who are not directly connected with its clandestine purposes.

Closed cities are a feature of heavily militarized countries and secretive regimes, and many still exist in the successor countries to the Soviet Union. In modern Russia, such places are officially known as "closed administrative-territorial formations" (закрытые административно-территориальные образования, zakrytye administrativno-territorial'nye obrazovaniya, or ЗАТО ZATO for short).

Sometimes closed cities may only be represented on classified maps which are not available to the general public. In some cases there may be no road signs or directions to closed cities, and they are usually omitted from railroad time tables and bus routes.

Sometimes closed cities may be indicated obliquely as a nearby insignificant village, with the name of the stop serving the closed city made equivocal or misleading. For mail delivery, a closed city is usually named as the nearest large city and a special postcode e.g. Arzamas‑16, Chelyabinsk‑65. The actual settlement can be rather distant from its namesakes; for instance, Sarov, designated Arzamas-16, is in the federal republic of Mordovia, whereas Arzamas is in the Nizny Novgorod region (roughly 75 kilometres (47 mi) away). People not dwelling in a closed city were subject to document checks and security checkpoints, and explicit permission was required for them to visit. To relocate to the closed city, one would need security clearance by the KGB.

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