St. Thomas Island (Bulgarian: остров св. Тома, ostrov sv. Toma) or Snake Island (Змийски остров, Zmiyski ostrov) is a Bulgarian island in the Black Sea, 15 km south of Sozopol. It has an area of 3,000 m²  and is the only place in Bulgaria where wild cacti grow, brought from Bratislava and planted on the orders of Tsar Boris III in 1933. They have covered most of the island since then.
St. Thomas Island is named after a chapel dedicated to Saint Thomas that once existed on it. Snake Island, the alternate name, refers to the abundant grey water snakes that inhabit it, feeding on fish.