Here, we report the electronic structures at the N, N′-dipentyl-3,4,9,10-perylenedicarboximide (PTCDI-C5)/CH3NH3PbI3 interface identified in-situ by X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. Strong chemical reactions are found to occur upon the deposition of PTCDI-C5 molecules on CH3NH3PbI3. Electron donation from PTCDI-C5 molecules to CH3NH3PbI3 leads to the filling of surface states and the emergence of an interfacial gap state with its onset tailed to the Fermi level. As a consequence, the downward surface band bending resulting from surface states acting as donor states at the pristine perovskite surface is reduced by 0.2 eV. After the energy level alignment at the interface is established, the perovskite conduction band minimum is found to be in line with the lowest unoccupied molecular orbital favoring the electron extraction with a moderate valence band maximum-highest occupied molecular orbital offset of ∼0.7 eV. The present results demonstrate that interfacial chemical reactions can dictate energetics at organic/perovskite interfaces. Understanding the chemical interaction and resultant electronic structures at those interfaces is crucial for efficient and long-term stable perovskite-based devices when passivation of chemical active sites and matched energy level could be readily reached.