We have systematically studied the effects of ultrathin platinum catalytic films in the sub-10 nm regime on the performance of the back-illuminated dye-sensitized solar cells using anodic TiO2 nanotube arrays. Results indicate that the maximum power conversion efficiency is achieved with a platinum film of 2.48 nm thickness. The charge transfer resistance sharply decreases from 6.39 Ω cm−2 (1.24 nm) to 0.24 Ω cm−2 (2.48 nm); however, subsequently, it keeps on increasing to 0.76 Ω cm−2 (3.72 nm) and 1.36 Ω cm−2 (5.58 nm); finally, it re-decreases to 0.25 Ω cm−2 (8.06 nm), and remains stable afterwards. The platinum film morphology and particle size on the counterelectrode have been demonstrated to play an important role in catalytic activity in the ultrathin thickness range. This study is meaningful from the aspect of improving the performance of the back-illuminated devices and reducing cost; and meanwhile sheds light on the main factors of influencing catalytic activity in the ultrathin catalytic films.