We developed an asymmetric fluorescent sensor 1 for Cu2+, based on 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY), by introducing 4-carboxyphenyl and bis(pyridin-2-ylmethyl)amine groups at the 5- and 3-positions, respectively, of the BODIPY core. We then investigated the photophysical and cation-sensing properties of the sensor. BODIPY 1 showed large absorption and fluorescence spectral shifts on binding to Cu2+. The fluorescence peak at 580 nm red-shifted to 620 nm. The binding stoichiometry of BODIPY 1 and Cu2+ was 1:3. The ratio of the fluorescence intensity at 620 nm to that at 580 nm (F620/F580) increased with increasing concentration of Cu2+ (3–10 equiv); this enabled ratiometric determination of Cu2+. Although BODIPY 1 showed good selectivity for Cu2+, there was an interfering effect of Fe3+. BODIPY 1 could be used for the naked-eye detection of Cu2+ in a water-containing sample.