Here, we report an alternative environmentally friendly room-temperature molten salt, methylammonium acetate (MAAc), as a novel solvent for facile fabrication of perovskite solar cells(PSCs) in ambient air. MAAc possesses excellent chemical properties along with high viscosity, negligible vapor pressure, and a nonhazardous nature. Complete solubility of both methylammonium and lead salts by hydrogen bonds in MAAc was observed. Dense and pinholefree perovskite films with high reproducibility can be readily prepared by a simple one-step method without an anti-solvent even under a relative humidity of over 80%. Under optimized processing conditions, we achieved an average power conversion efficiency of 18.42% and a maximum efficiency of 20.05% in CH3NH3PbI3-based planar heterojunction structure. In addition, devices without encapsulation remained above 93% of their original efficiency for more than 1,000 h in ambient air. These findings may open up the possibility of developing a new approach for further improving PSC performance with higher reproducibility and reliability in ambient atmosphere.