This paper describes a new species of marine endolithic cyanophyte, Cyanosaccus aegaeus Anagnostid is et Pantazidou . It differs from the type species C. piriformis Lukas et Golubic by its significantly smaller cells and endospores. It was found in marine carbonate substrates in the upper intertidal zone, on the east coast of the volcanic Island Thira (Santorini), Aegean Sea, Greece. Cyanosaccus is reported from Europe for the first time.