The island of Hainan (China) is located on the northern periphery of the subtropical Pacific Ocean in the South China Sea and possesses one of the most prominent fringing coral reefs of China. The marine algal flora of the island was described for the first time after several early expeditions in the 1930’s (and around 1960) mentioning 68 species of marine green algae (Chlorophyta) in total. Sixty years later, the island was revisited in autumn 1990 and spring 1992 within in the framework of two extensive expeditions in order to scrutinise the algal biodiversity of the coral reef habitats. Again 20 years later, three more surveys were performed in 2008, 2009 and 2010 focusing only on the southern shores of Hainan Island near Sanya city. The analysis of this material revealed the existence of 31 new records of marine green algae for Hainan Island, including 17 species which also constitute new records for China. The new records are described and characteristic details are depicted. Collection sites around Hainan Island and world-wide distributions are given. Thirteen newly recorded species were only sampled once and therefore seem to be rare. Thirteen other species are pre-dominantly epiphytic or endophytic. The new records for China are fairly typical for tropical to subtropical locations of south-western to south-eastern Asia, except Chlorochytrium cohnii (distribution hitherto restricted to Europe), Chaetomorpha minima (distribution hitherto restricted to North and Central America, Africa, Indian Ocean islands) and Cladophora perpusilla (distribution hitherto restricted to Central and South America, Australia, New Zealand, Pacific islands). None of the new records are considered to represent an invasive species.