Surface modification of martensitic stainless steel AISI 420 was achieved using friction stir processing (FSP) with a W–Re pinless tool. The microstructure, hardness and corrosion resistance of the FSPed 420 fabricated at various traverse speeds (150 to 250 mm/min) of the tool were investigated and compared with annealed and conventionally hardened 420. The depth and width of the FSPed zone decrease as the traverse speed increases due to decrease in heat input. The volume fraction of retained austenite at the center and advancing side of the FSPed zone increases with the traverse speed attributed to the high cooling rate. The hardness (up to 697 HV) and corrosion resistance in 3.5 wt% NaCl solution at 25 °C of the FSPed specimens are higher than that of the conventionally hardened one.