A complete checklist of intertidal to shallow subtidal marine green seaweeds (Chlorophyta) growing on the subtropical island of Hainan (China) is presented here for the first time. It covers data from extensive recent (1990-2009) and historical (1933-1935) collections, and additional published records from various time periods. Data were analyzed by time period. We postulate that environmental changes on Hainan Island documented since the 1980s (e.g., degradation of coral reefs, development of tourism and mariculture farms) are reflected in the green algal species complement and in the dominance or absence of specific algal groups during different time periods. In total, 105 green algal taxa were recorded, including 37 new to Hainan Island, and 18 new records for China. There was a clearly evident change in floristic composition between early and recent collections. In the 1930s, there was a dominance of Caulerpaceae, Codiaceae and Cladophoraceae. By 1990/1992, the numbers of Ulvaceae had increased 1.6-fold and numbers of Cladophoraceae 1.7-fold. Both families contain many opportunistic species that prefer nutrient-enriched or degraded environments. At the same time, species richness of Codiaceae, Caulerpaceae and Udotaceae, families with complex thallus structures, decreased considerably. The floristic differences between the 1990/1992 and 2008/2009 collections were minor. © 2011 by Walter de Gruyter Berlin Boston.