  CO$_2$ photoreduction on photocatalysts is a nature-friendly solution to
decrease the CO$_2$ amount, but the method still has low efficiency because of
difficult separation and easy recombination of charge carriers in available
catalysts. In this study, a high-entropy oxynitride was introduced as an active
photocatalyst for photoreduction. The material had a chemical composition of
TiZrNbHfTaO$_6$N$_3$ and was produced by a high-pressure torsion method
followed by oxidation and nitriding. It showed higher photocatalytic CO$_2$ to
CO conversion compared to corresponding high-entropy oxide, benchmark
photocatalyst P25 TiO$_2$, and almost all catalysts introduced in the
literature. The high activity of this oxynitride, which also showed good
chemical stability, was attributed to the large absorbance of light and easy
separation of electrons and holes, the low recombination of charge carriers,
and the high CO$_2$ adsorption on the surface. These findings introduce
high-entropy oxynitrides as promising photocatalysts for CO$_2$ photoreduction.
