<p>Additively manufactured (AM) austenitic stainless steel (ASS, e.g., 316LSS) potentially exhibits excellent strength-ductility synergy in which the deformation-induced martensitic transformation (DIMT) is decisive. However, the DIMT mechanism is still elusive for AM 316LSS. Here we decipher the role of twin boundary (TB) and grain boundary (GB) in governing the DIMT behavior as well as the associated atomic-scale mechanism by characterization-informed atomistic simulations. Experimental characterizations of DIMT in AM 316LSS show martensite distributed near TBs under quasi-static (QS) tension, but closely related to GBs under high strain rate (HSR) tension. Informed by characterizations, atomistic models covering grain sizes, GB angles and TBs are then constructed to reveal the effect of GBs and TBs on DIMT behavior. It is found that the low-angle GB (LAGB) and small grain size in AM 316LSS suppress DIMT, whereas the synergistic effect of high-angle GB (HAGB) and large grain size (e.g., in wrought 316LSS) results in large-area DIMT. When TBs exist in the 316LSS grains, TBs can promote intragranular DIMT to make DIMT independent of GB angle and grain size, agreeing with DIMT observed in both wrought and AM 316LSS under QS tension. This is ascribed to the TBs-nearby heavy strain concentration that easily results in DIMT behavior and the TBs-nearby atoms that satisfy the Nishiyama-Wasserman relationship for triggering DIMT nucleation within the grain. In contrast, HAGBs dominate DIMT behavior in models without TBs owing to the GBs-nearby local lattice distortion that satisfies the Kurdjumov-Sachs relationship for allowing phase transformation. There are almost no HAGBs and thus an ignorable DIMT in AM 316LSS, agreeing with the experimental HSR tension results. These findings should shed light on the DIMT mechanism in AM 316LSS and help the design of AM 316LSS with improved mechanical performance.</p>