In plants, environmental adversity often leads to the formation of reactive oxygen species (ROS) because of excess light energy. We found that incubation of rice leaves under high light intensity together with CO2-depleted air seriously damaged photosynthesis; however, the application of potassium formate (2 mM) before the photoinhibitory treatment protected the photosynthesis of the plant from photoinhibition. Further analysis revealed that formate protects PSII, RuBisCO, and FBPase from photoinhibition. Formate is thought to be involved in endogenous radical scavenging and/or in the supply of CO2, derived from the formate, thereby reducing oxidative damage to the photosystems under photoinhibitory conditions.