Tin halide perovskites make up the only lead-free material class endowed with optoelectronic properties comparable to those of lead iodide perovskites. Despite significant progress, the device efficiency and stability of tin halide perovskites are still limited by two potentially related phenomena, i.e., self-p-doping and tin oxidation. Both processes are likely related to defects; thus, understanding tin halide defect chemistry is a key step toward exploitation of this class of materials. We investigate the MASnI3 perovskite defect chemistry, as a prototype of the entire materials class, using state-of-the-art density functional theory simulations. We show that the inherently low ionization potential of MASnI3 is solely responsible of the high stability of tin vacancy and interstitial iodine defects, which are in turn at the origin of the material p-doping. Tin vacancies create a locally iodine-rich environment that could promote Sn(II) → …