Display omitted •A simple and versatile one-step process for black TiO2-x microspheres is demonstrated.•The process is based on an ultrasonic spray pyrolysis under oxygen-deficient environment.•The existence of Ti3+ and oxygen vacancies in the microsphere is confirmed.•The microsphere shows distinct absorption in the visible region.•The black TiO2-x microspheres show a higher photocatalytic activity than its white form. Black TiO2-x has recently emerged as one of the most promising visible-light-driven photocatalysts, but current synthesis routes that require a reduction step are not compatible with cost-effective mass production and a relatively large particle such as microspheres. Herein, we demonstrate a simple, fast, cost-effective and scalable one-step process based on an ultrasonic spray pyrolysis for the synthesis of black TiO2-x microspheres. The process utilizes an oxygen-deficient environment during the pyrolysis of titanium precursors to directly introduce oxygen vacancies into synthesized TiO2 products, and thus a reduction step is not required. Droplets of a titanium precursor solution were generated by ultrasound energy and dragged with continuous N2 flow into a furnace for the decomposition of the precursor and crystallization to TiO2 and through such a process spherical black TiO2-x microspheres were obtained at 900 °C. The synthesized black TiO2-x microsphere with trivalent titanium/oxygen vacancy clearly showed the variation of physicochemical properties compared with those of white TiO2. In addition, the synthesized microspheres presented the superior photocatalytic activity for degradation of methylene blue under visible light irradiation. This work presents a new methodology for a simple one-step synthesis of black metal oxides microspheres with oxygen vacancies for visible-light-driven photocatalysts with a higher efficiency.