The Snake Island or St. Thomas is an island of Bulgaria in the Black Sea.

Since 1962 the island is part of the Ropotamo Reserve, Primorsko Municipality. It is located 36 km from Bourgas.

The Snake Island has been named after the many gray water snakes that inhabit it and feed on fish in the waters around it.

The island is located north of the mouth of the Ropotamo River (opposite Arkutino) on an area of twelve acres.

It is one of the places in Bulgaria, where wild cactuses meet. They were planted in 1933 by King Boris III, who brought them from the botanical garden in Bratislava. Cactuses are two different species of the sort Opuntia, spread about half of the territory of Snake Island and make it difficult to pass through.

They are especially beautiful in June, when they are blooming with large, yellow flowers. Cactuses create fruits, and their fruits (August - September), with a size of blue plum and strawberry flavor, can be eaten.