  Oxides with perovskite-based structures have been known as essential
materials for fascinating phenomena such as high-temperature and unconventional
superconductivity. Discoveries of these oxide superconductors have driven the
science community to vastly extend the concepts of strongly correlated electron
systems. The base of these materials, the cubic perovskite oxides, $AB$O$_3$,
also exhibit superconductivity with $T_{\mathrm{c}}$ of up to 30 K, as reported
for Ba$_{0.6}$K$_{0.4}$BiO$_3$. Perovskite oxides have their counterparts,
antiperovskite oxides $A_3B$O (or "$B$O$A_3$"), in which the position of metal
and oxygen ions are reversed and therefore metallic $B$ ions take unusual
negative valence states. However, no superconductivity has been reported among
antiperovskite oxides. Here, we report the discovery of the first
superconducting antiperovskite oxide Sr$_3$SnO with $T_{\mathrm{c}}$ of around
5 K. Sr$_3$SnO possesses Dirac points in its electronic structure, originating
from the inversion of bands with different parities. Based on theoretical
analysis, we propose possibility of topological odd-parity superconductivity
analogous to the superfluid $^3$He-B, in moderately hole-doped Sr$_3$SnO,
originating from unusual orbital texture on the Fermi surface. We envision that
this discovery of a new class of oxide superconductor with the inverted valence
configuration will stimulate the exploration of topological materials science
based on a variety of antiperovskite oxides.
