Kronstadt also spelled Kronshtadt, Cronstadt or Kronštádt (German: Krone for "crown" and Stadt for "city"; Finnish: Retusaari, Estonian: Kroonlinn), is a municipal town in Kronshtadtsky District of the federal city of Saint Petersburg, Russia, located on Kotlin Island, 30 kilometers (19 mi) west of Saint Petersburg proper near the head of the Gulf of Finland.

It is also Saint Petersburg's main seaport. In March 1921, it was the site of the Kronstadt rebellion.

Traditionally, the seat of the Russian admiralty and the base of the Russian Baltic Fleet were located in Kronstadt guarding the approaches to Saint Petersburg. The historic centre of the city and its fortifications are part of the World Heritage Site Saint Petersburg and Related Groups of Monuments.

Kronstadt has been a place of pilgrimage for Orthodox Christians for many years due to the holy memory of Saint John of Kronstadt. Bus and water tours to Kronstadt are taken daily from Saint Petersburg.

Kronstadt was the birthplace of Pyotr Kapitsa, co-recipient of the 1978 Nobel Prize in Physics.