The structural effects of substrates on the incident photon-to-current conversion efficiency (IPCE) of Zn porphyrin (ZnP) dyes (ZnP-ref, YD2, and ZnPBAT) have been studied on well-defined single-crystal surfaces of rutile TiO 2 (TiO 2 (111), TiO 2 (100), and TiO 2 (110)). IPCE of ZnP-ref depends on the structure of the substrates remarkably: TiO 2 (100) < TiO 2 (110) < TiO 2 (111). IPCE of ZnP-ref/TiO 2 (111) is 13 times as high as that of ZnP-ref/TiO 2 (100) at 570 nm. YD2 and ZnPBAT also give the highest IPCE on TiO 2 (111). The relative coverages of the porphyrin dyes give the following order: TiO 2 (111) < TiO 2 (110) < TiO 2 (100). This order is opposite to that of IPCEs. The orientation of the dyes is predicted using density functional theory calculations on simplified models of TiO 2 surfaces. The highest IPCE on TiO 2 (111) is attributed to the high rate of electron transfer through the space due to the fluctuation of the tilt angle of the adsorbed dyes.