Photocatalytic CO₂ conversion is a clean technology to deal with CO₂ emissions, and titanium oxide (TiO₂) polymorphs are the most investigated photocatalysts for such an application. In this study, black TiO₂ brookite is produced by a high-pressure torsion (HPT) method and employed as an active photocatalyst for CO₂ conversion. Black brookite with a large concentration of lattice defects (vacancies, dislocations and grain boundaries) showed enhanced light absorbance, narrowed optical bandgap and diminished recombination rate of electrons and holes. The photocatalytic activity of the black oxide for CO₂ conversion was higher compared to commercial brookite and benchmark P25 catalyst powders. First-principles calculations suggested that the presence of oxygen vacancies in black brookite is effective not only for reducing optical bandgap but also for providing active sites for the adsorption of CO₂ on the surface of TiO₂.