Naissaar is an island in Estonia. It is located in the Gulf of Finland, northwest of the capital city Tallinn, and is administratively part of Viimsi Parish. The island has an area of 18.6 square kilometres. It is 8 kilometres long and 3.5 kilometres wide, and lies about 8.5 kilometres from the mainland. The highest point on the island is Kunilamägi, which is 27 metres above sea level. The island consists predominantly of coniferous forest and piles of stones and boulders. In 2020, the island had a population of 17; in 2011 the island had about 35 permanent residents and some summer residents.

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.