We have developed efficient coumarin-dye-sensitized nanocrystalline TiO₂ solar cells (DSSCs). We have attained a 7.5% solar energy-to-electricity conversion efficiency, the highest performance among DSSCs based on organic-dye photosensitizers, under AM 1.5 irradiation (100 mW cm^-2) with a DSSC based on a novel coumarin-dye photosensitizer and nanocrystalline TiO₂ photoelectrode. Our result strongly indicates that the molecular design of organic-dye photosensitizers for DSSCs can be successful and that the prospects for application of these photosensitizers in DSSCs are promising.