Gulf of Finland National Park is located outside of the inner archipelago. The landscape is dominated by groups of islands, which also include quite large forested islands. Behind the open sea other large forested islands can be seen, but they are outside the Finnish borders.
Most of the islands in Gulf of Finland National Park are steep-sided and bare rocky islets washed by the waves. This is typical for the southern coast of Finland, because the bedrock, reddish brown rapakivi granite, grinded by the Ice Age and the sea, splits into cubic forms. Between the boulders grow thickets of bushes. At some places the ice and waves have made embankments out of the cobble stones and gravel, where the terraces are full of yellow flowers in the beginning of the summer. Often on these gravel fields nests a large community of Arctic Terns (Sterna paradisaea). In the winter, the ice on the Gulf of Finland is an important area for the reproduction of the Grey Seal (Halichoerus grypus) and Baltic Seal (Phoca hispida botnica).
The vegetation on the forested islands usually consists of low, sparse pine forests and reindeer lichen (Cladonia rangiferina), and rooted in the cracks of the rock grows the Common Juniper (Juniperus communis). In the inner parts of Ulko-Tammio Island and the other larger islands you can find surprisingly lush southern herb-rich forests. The most luxuriant of the islands is Ristisaari Island with its Common Ash (Fraxinus excelsior) groves, flowering dry meadows and coastal meadows. The island was formerly used as a pasture by the inhabitants of Kaunissaari Island.
In contrast to the rocky islands, there is Pitkäviiri Ridge, a 2-km-long esker island which has risen from the sea. For a long time, its sandy beaches and large shoals have been a popular destination for sunny day trips in the summer.
Gulf of Finland National Park is famous for its set of sea birds, and there are many good islands for birds in the area. The most common bird species include the Goosander (Mergus merganser) and the Tufted Duck (Aythya fuligula). On the islets where landing is restricted, can even the more demanding birds of the open sea find a peaceful place for nesting, for example the Razorbill (Alca torda) and the Black Guillemot (Cepphus grylle). Also the Velvet Scoter (Melanitta fusca) which has declined strongly elsewhere in Finland, still nests in the park. Other sea birds which are few in number, such as the Guillemot (Uria aalge), the Greylag Goose (Anser anser) and the Rock Pipit (Anthus petrosus), can also be seen. The Barnacle Goose (Branta leucopsis) is a new rare and interesting nesting bird in the park area.
The low salinity of the brackish water restricts the distribution of many species in the Gulf of Finland. For example the Common Eider (Somateria mollissima), which is the dominant species of the rest of the Baltic Sea, is few on the eastern Gulf of Finland compared to the western parts, because its mainly eats the Common Mussel (Mytilus edulis), which is small in size and numbers in these parts. Another dominant species of the Baltic Sea, the Bladder-wrack (Fucus vesiculosus) lives in the National Park at the extreme limits of its distribution.
The end of May is the best time for bird watchers. Hundreds of thousands arctic water birds and waders pass through the area on their way to their nesting places in the north. The most abundant migrating birds are the Long-tailed Duck (Clangula hyemalis), the Black Scoter (Melanitta nigra), the Velvet Scoter (Melanitta fusca), The Barnacle Goose (Branta leucopsis), the Brent Goose (Branta bernicla) and the Dunlin (Calidris alpina).