Developing high-performance, low-cost, and conductive nonprecious electrocatalysts for the oxygen reduction reaction (ORR) has been a key challenge for advancing fuel cell technologies. Here, we report on a novel family of cobalt nitrides (CoxN/C, x = 2, 3, 4) as ORR electrocatalysts in alkaline fuel cells. Co4N/C exhibited the highest ORR activity among the three types of cobalt nitrides studied, with a half-wave potential (E-1/2) of 0.875 V vs RHE in 1 M KOH, rivaling that of commercial Pt/C (0.89 V). Moreover, Co4N/C showed an 8-fold improvement in mass activity at 0.85 V, when compared to cobalt oxide, Co3O4/C, and a negligible degradation (Delta E-1/2 = 14 mV) after 10?000 potential cycles. The superior performance was ascribed to the formation of a conductive nitride core surrounded by a naturally formed thin oxide shell (about 2 nm). The conductive nitride core effectively mitigated the low conductivity of the metal oxide, and the thin oxide shell on the surface provided the active sites for the ORR. Strategies developed herein represent a promising approach for the design of other novel metal nitrides as electrocatalysts for fuel cells.