In the present study, the influence of multi-pass friction stir processing (FSP) of AISI 420 martensitic stainless steel on the microstructure, hardness, and corrosion behavior was investigated. Similar to single-pass FSPed 420, the multi-pass FSPed specimens with different overlapping ratios incurred microstructural change at the center and retreating side of the second track with martensite, retained austenite, and chromium carbides. Overlapping of the two successive tracks in the multi-pass FSPed 420 led to back-tempering and a local drop in hardness at the advancing side of the second track. The precipitation of chromium carbides in the tempered regions of the multi-pass FSPed specimens became the active sites for preferential corrosion attack in the 3.5 wt.% NaCl solution at 25 ◦C.