Ultrathin perovskite oxides with tailored crystal structures are promising catalysts for oxygen evolution reaction (OER) owing to their high intrinsic catalytic activity and large exposed active surface area. However, the synthesis of phase‐controllable perovskite oxide nanosheets with thickness down to a few nanometers remains a challenge since the formation of a perovskite phase often requires long‐time calcination at high temperatures. Here, a salt‐templated strategy for fabrication of atomically thin perovskite oxide of LaMnO₃ with tailored phase structure for highly active OER catalysts is reported. The orthorhombic structure of LaMnO₃ nanosheets demonstrates much higher electrochemical activity than the tetragonal or hexagonal phase and the benchmark IrO₂ catalyst, exhibiting extremely small onset overpotential (≈70 mV) and a low overpotential (≈324 mV at 10 mA ) in alkaline solution. The remarkable OER activity of this catalyst is attributed to the desired surface binding energetics (or the unique electronic structures) inherent to the orthorhombic phase, as predicted by density functional theory calculations and confirmed by experimental measurements. Further, it is believed that this study paves a new path toward rational the design of perovskite oxide nanosheets with desired phase structures for many applications.