Kronstadt (Russian: Кроншта́дт, romanized: Kronshtadt, IPA: [krɐnˈʂtat]) is a Russian port city in Kronshtadtsky District of the federal city of Saint Petersburg, located on Kotlin Island, 30 km (19 mi) west of Saint Petersburg, near the head of the Gulf of Finland. It is linked to the former Russian capital by a combination levee-causeway-seagate, the St Petersburg Dam, part of the city's flood defences, which also acts as road access to Kotlin island from the mainland.

Founded in the early 18th century by Peter the Great, it became an important international centre of commerce whose trade role was later eclipsed by its strategic significance as the primary maritime defence outpost of the former Russian capital. The main base of the Russian Baltic Fleet was located in Kronstadt, guarding the approaches to Saint Petersburg. In March 1921, the island city was the site of the Kronstadt rebellion.

The historic centre of the city and its fortifications are part of the World Heritage Site that is Saint Petersburg and Related Groups of Monuments.

Kronstadt has been a place of pilgrimage for Orthodox Christians for many years due to the memory of Saint John of Kronstadt.

The name of the city is alternatively spelled as Kronshtadt, Cronstadt or Kronštadt. It is derived from German Kronstadt (lit. 'crown city'). The district is referred to as Kronshtadtsky District.

Kronstadt was founded by Peter the Great, whose Imperial Russian forces took the island of Kotlin from the Swedes during the Great Northern War in 1703. The first fortifications were inaugurated on 18 May [O.S. 7 May] 1704. These fortifications, known as Kronshlot [ru] (Кроншлот), were constructed very quickly. During the winter, the Gulf of Finland freezes over completely. Under the command of Governor-general Alexander Danilovich Menshikov, workers used thousands of frames (caissons) made of logs from evergreen trees filled with stones which were moved by horses across the frozen sea, and placed in cuttings made in the ice. Thus, several new small islands were created, and forts were erected on them, virtually closing access to Saint-Petersburg by sea. Only two narrow navigable channels remained, with forts guarding them. On 8 July 1704, the fortress was attacked by a Swedish fleet led by Jacob de Prou.

One of the first governors of Kronstadt was a veteran of the Royal Scots Navy, Admiral Thomas Gordon who was a refugee in Russia from the Scottish union with England and became chief commander of the port of Kronstadt from 1727 to 1741.

Just as Kronstadt became populated and fortified, it attracted merchants from maritime powers most notably, the Dutch, the British and the Germans through the old Hanse connections. The community of British merchants or "factors" came to be known as the English Factory, despite the fact that many of them were Scots. They settled both in Kronstadt and in St Petersburg itself and for a time dominated both inward and outward trade, especially in the reign of Catherine the Great. They became an integral part of British trade and foreign policy through the Board of Trade in London. A number of the British settlers became naturalised Russians. The population by the 1840s was 44,000. Trading alliances were sharply interrupted by the outbreak of the Crimean War (1854).