Discovery of layered superconductors such as cuprates and iron-based compounds has unveiled new science and compounds. In these superconductors, quasi two-dimensional layers including transition metal cations play principal role in the superconductivity via carrier doping by means of aliovalent-ion substitution. Here, we report on a two-dimensional superconductivity at 2 K in ThCr2Si2-type layered oxide Y2O2Bi possessing conducting monatomic Bi2- square net, possibly associated with an exotic superconductivity. The superconductivity emerges only in excessively oxygen-incorporated Y2O2Bi with expanded inter-net distance, in stark contrast to non superconducting pristine Y2O2Bi reported previously. This result suggests that the element incorporation into hidden interstitial site could be an alternative approach to conventional substitution and intercalation methods for search of novel superconductors.