Additively manufactured (AM) 316L steel exhibits extraordinary high yield strength, and surprisingly good ductility despite the high level of porosity in the material. This detailed study sheds light on the origins of the observed high yield strength and good ductility. The extremely fine cells which are formed because of rapid cooling and dense dislocations are responsible for the macroscopically high yield strength of the AM 316L (almost double of that seen in annealed 316L steel). Most interestingly, twinning is dominant in deformed samples of the AM316. It is believed that twinning-induced plasticity (TWIP) behaviour to be responsible for the excellent ductility of the steel despite the high level of porosity. The dominant twinning activity is attributed to Nitrogen gas used in 3D printing. Nitrogen can lower the stacking fault energy of the steel, leading to the disassociation of dislocations, promoting the deformation twinning. Twinning induces large plasticity during deformation that can compensate the negative effect of porosity in AM steel. However, twinning does not induce significant hardening because (1) the porosity causes a negative effect on hardening and (2) twinning spacing is still larger than extremely fine solidification cells.