Friction-stir processing (FSP) has been attempted to harden the surface of a high-carbon martensitic stainless steel (AISI 440C) and to improve its corrosion resistance. Owing to high input of heat, the penetration depth of the FSPed zone increases in the range from 1.5 to 2 mm when the translational speed decreases. As the translational speed increases, higher content of retained austenite in the advancing side and center of the processed zone are obtained due to higher cooling rate. The maximum hardness of the FSPed 440C is up to 779 HV1 and higher than that of the conventionally hardened sample austenitized at 1200 ℃ (618 HV1). The pitting corrosion resistance of the sample processed at 300 mm·min−1 in 3.5 wt% sodium chloride solution is the lowest among the FSPed samples. It is mainly attributed to the heterogeneous microstructures at the retreating side (martensite and carbides) and advancing side and center (with martensite and retained austenite) leading to selective attack occurred at the more active region, i.e. the retreating side.