•Iodine and Fe-Nx were simultaneously introduced into carbon (FeNI-CNS) by a solution plasma process.•Iodine dopants contribute to the oxygen reduction reaction rate with a higher current density.•A synergistic effect further enhances the catalytic activity of the FeNI-CNS.•FeNI-CNS exhibits better durability than commercial Pt/C catalysts. The oxygen reduction reaction (ORR) plays a critical role at the cathode of fuel cells and metal–air batteries. In this study, we report the one-pot synthesis of iodine- and Fe-Nx impregnated carbon nanospheres (FeNI-CNS) using a solution plasma process and their effect on electrocatalytic activity. FeNI-CNS exhibits a fast four-electron transfer reaction with a high current density and superior durability in both alkaline and acidic media. The excellent catalytic performance is attributed to the synergetic effect between the iodine and Fe-Nx sites, which provide a higher reaction rate and faster ORR, respectively. We believe that these findings represent a new approach for enhancing ORR catalytic performance and catalyst synthesis.