Minimizing reverse bias dark current density (Jdark) while retaining high external quantum efficiency is crucial for promising applications of perovskite photodiodes, and it remains challenging to elucidate the ultimate origin of Jdark. It is demonstrated in this study that the surface defects induced by iodine vacancies are the main cause of Jdark in perovskite photodiodes. In a targeted way, the surface defects are thoroughly passivated through a simple treatment with butylamine hydroiodide to form ultrathin 2D perovskite on its 3D bulk. In the passivated perovskite photodiodes, Jdark as low as 3.78 × 10‐10 A cm‐2 at ‐0.1 V is achieved, and the photoresponse is also enhanced, especially at low light intensities. A combination of the two improvements realizes high specific detectivity up to 1.46 × 1012 Jones in the devices. It is clarified that the trap states induced by the surface defects can not only raise the generation‐recombination current density associated with the Shockley–Read–Hall mechanisms in the dark (increasing Jdark), but also provide additional carrier recombination paths under light illumination (decreasing photocurrent). The critical role of surface defects on Jdark of perovskite photodiodes suggests that making trap‐free perovskite thin films, for example, by fine preparation and/or surface engineering, is a top priority for high‐performance perovskite photodiodes. This work demonstrates that the trap states induced by surface iodine vacancies significantly contribute to the generation‐recombination current in the dark and thus to the reverse bias dark current of perovskite photodiodes. The insights into the ultimate origin of the dark current in perovskite photodiodes will be of great interest in perovskite electronics and optoelectronics.