St. Thomas Island (Bulgarian: остров св. Тома, ostrov sv. Toma) or Zmiyski ostrov (Змийски остров, Snake island), is a Bulgarian island in the Black Sea.
It is situated in Burgas Province, Primorsko Municipality, 7 km (4 mi) north of Primorsko and 15 km (9 mi) km south of Sozopol.