Although the efficiency of perovskite solar cells (PSCs) is close to crystalline silicon solar cells, the instability of perovskite, especially in humid condition, still hinders its commercialization. As an effective method to improve their stability, surface functionalization, by using hydrophobic molecules, has been extensively investigated, but usually accompanied with the loss of device efficiencies owing to their intrinsic electrical insulation. In this work, for the first time, it is demonstrated that 3‐alkylthiophene‐based hydrophobic molecules can be used as both water‐resistant and interface‐modified layers, which could simultaneously enhance both stability and performance significantly. Benefitting from their unique structures of thiophene rings, the π‐electrons are highly delocalized and thus enhance the charge transfer and collection at the interface. The device based on 3‐hexylthiophene treatment exhibits a champion energy conversion efficiency of 19.89% with a dramatic 10% enhancement compared with the pristine one (18.08%) of Cs_0.05 FA_0.81 MA_0.14 PbBr_0.45 I_2.55‐based PSCs. More importantly, the degradation of the long‐term efficiency of unsealed device is less than 20% in Cs_0.05 FA_0.81 MA_0.14 PbBr_0.45I_2.55‐based PSCs after more than 700 h storage in air. This finding provides an avenue for further improvement of both the efficiency and stability of PSCs.