High pressure torsion (HPT) at room temperature was used for post-treatment of additively manufactured Fe–Cr–Ni stainless steel with 12.9 wt % Ni as a very strong austenite stabilizer. The results showed that HPT caused a considerable increase in nanohardness of the additively manufactured samples. In contrast with thermodynamic equilibrium-state modeling, a phase transformation from FCC to HCP structure occurred, leading to the formation of ε-martensite during HPT on high angle boundaries, low angle boundaries, and dislocation cells with no detection of deformation twins. It was demonstrated that the combination of additive manufacturing thanks to the high density of dislocations after solidification and HPT process expands the opportunities of both methods to control deformation mechanisms in stainless steels leading to different phase and microstructural features. Thus, the outcome of this study provides a fundamental basis to design advanced structural materials.