Fort Alexander, also Fort Alexander I, or Plague Fort is a naval fortress on an artificial island in the Gulf of Finland near St. Petersburg and Kronstadt. From 1899 to 1917, the fort housed a research laboratory on plague and other bacterial diseases.

The Gulf of Finland is the easternmost arm of the Baltic Sea. It extends between Finland to the north and Estonia to the south, to Saint Petersburg in Russia to the east, where the river Neva drains into it. Other major cities around the gulf include Helsinki and Tallinn. The eastern parts of the Gulf of Finland belong to Russia, and some of Russia's most important oil harbors are located farthest in, near Saint Petersburg. As the seaway to Saint Petersburg, the Gulf of Finland has been and continues to be of considerable strategic importance to Russia. Some of the environmental problems affecting the Baltic Sea are at their most pronounced in the shallow gulf. Proposals for a tunnel through the gulf have been made.