Horizontal and vertical distribution of macroalgae were surveyed during the summer in Lake Nakaumi, a brackish lagoon. In the summers of 2004–2006 water conditions （transparency and salinity）and macroalgal assemblage characteristics（percent cover, species richness, and depth limit）were surveyed at 15 coastal sites. Both species richness and depth limit decreased with increasing the distance from the seawater inlet. However, the patterns of algal cover differed among green, red, and brown algae: green algae grew in all areas in the lake. In contrast, brown algae was dominant only in the area closest to the sea. Red algae grew in all areas except that most distant from the sea. Since species richness and depth limit were correlated with transparency, and transparency was correlated with the distance from sea but not salinity, transparency seemed to be the primary cause of the observed spatial pattern. In contrast, because the patterns of algal cover do not correlate with either transparency or salinity, we suggest that other factors, including grazing by animals and competition among algae, were important in establishing the observed patterns. Finally, a schematic diagram of macroalgal distribution patterns is presented.