We have investigated the adsorption stability of ruthenium N749 dye [black dye (BD)], a highly efficient dye for dye-sensitized solar cells (DSCs), through protonated and deprotonated carboxyl group anchors on a TiO2 anatase (101) surface by using first-principles calculations. Geometry optimizations of the surface system with a supercell and the UV–visible spectrum calculation of the optimized dye structure were carried out. Among the configurations with one and two anchors, the BD adsorption anchored with one protonated carboxyl group was found to be the most stable, in contrast to most previous reports. Hydrogen bonding between the proton retained in BD and the surface oxygen is responsible for the stability of the protonated anchor. We confirmed that the calculated UV–visible spectrum of the most stable dye structure shows the best consistency with the experimental data. It is also demonstrated that the electronic density of states largely depends on the proton position. This novel aspect of adsorption via a protonated carboxyl anchor gives a new perspective for interfacial electronic processes of DSCs.