Slow strain rate tests on hydrogen-containing specimens of PH13–8Mo maraging stainless steel showed H-assisted subcritical quasi-cleavage cracking, accelerating material failure. Fractographic analysis revealed that quasi-cleavage is composed of flat brittle areas and rougher areas. Electron backscatter diffraction (EBSD) was performed on a secondary subcritically grown crack. High resolution EBSD showed significant crystal lattice rotation, hence consequential plastic deformation, concentrated between the main crack tip and the cracks located ahead of it. Quasi-cleavage consisted of {100} cleavage cracks connected by ductile ridges, suggesting a discontinuous mechanism, involving re-initiation of new cleavage cracks ahead the main crack tip.