The impact of Ultra High Pressure Water Jet (UHPWJ) treatments on the near-surface microstructure of type 304L stainless steel as used in the nuclear industry is investigated. Samples water jetted at a 45° incident angle developed tensile surface residual stress; a 90° incident angle resulted in compressive residual stress. Both incident angles caused a reduction in grain size in the near-surface microstructure. The 45° treatment also resulted in an increase in surface roughness and a higher net mass loss from the steel. Assessment of the stainless steel chemical composition supported removal of the chromium rich surface passive film.