We studied the levels of importance of water regime components for plant community composition, hygrophilous character and species diversity in semi-natural wet grasslands. We also investigated to what extent the water level in adjacent canals drives the water regime in grasslands in two marshes of the Marais poitevin region (Atlantic coast of France). The species composition of homogenous plant communities, hygrophilous character and diversity were described from frequency data. The water regime was characterized by flooding duration, soil aeration deficit and soil dryness, and soil conductivity measured in the sub-halophilous marsh. Plant species composition and hygrophilous character were well predicted by the combined effects of all water regime components; flooding duration and dryness were most important, while soil water conductivity had a significant but limited impact. Diversity was poorly predicted by the environment, with a weak negative pattern related to flooding duration. The elevation contrasts within grasslands greatly contributed to species beta diversity. Different water regime components act as complementary drivers of plant communities in wet grasslands and need to be monitored, among other local environmental conditions. The importance of dryness stress is underlined, despite the dryness tolerance of many wet grassland species in relation to natural alternating wet/dry periods. Water level management in canals was found insufficient on its own to restore or maintain long flooding conditions in grasslands, probably because of its limited relationship with water regime in wet grasslands.