Abstract The coastal sand dune habitat can be characterized as a high sunlight habitat with seasonally high air and substrate temperatures, especially at midday during the growing season. Because there is little to no canopy cover, native sand dune plants are frequently exposed to high solar irradiance levels, which may result in the absorbance of more sunlight energy than can be dissipated through photosynthesis and transpiration, and lead to increased leaf temperature and/or decreased photosynthetic efficiency (i.e., photoinhibition). Several studies suggest that leaf inclination from horizontal functions in high sunlight habitats to regulate diurnal patterns of leaf-level sunlight incidence, thus avoiding the potential negative effects associated with excess sunlight absorbance. The aim of the present study was to investigate the function of leaf inclination in the coastal sand dune herb Hydrocotyle bonariensis Comm. ex Lam. Specifically, diurnal leaf-level incident sunlight, daily diurnal leaf temperature, and diurnal photosystem II efficiency (measured as Fv/Fm; proxy for photoinhibition) were compared between naturally inclined leaves and leaves restrained in a horizontal orientation during a growing season. Results showed that leaf inclination both reduced the total daily amount of sunlight incidence and altered the diurnal distribution pattern of sunlight incidence on leaf surfaces, leading to lower midday leaf temperatures and significantly less midday photoinhibition in control inclined leaves compared to experimental horizontal leaves. These results suggest that leaf inclination plays an important role in regulating leaf-level incident sunlight and leaf temperatures, thus reducing midday photoinhibition, for H. bonariensis in the coastal sand dune habitat.