Abstract The increasing need for sustainable energy and the transition from a linear to a circular economy pose great challenges to the materials science community. In this view, the chance of producing efficient nanocatalysts for water splitting using industrial waste as starting material is attractive. Here, we report low-cost processes to convert Mo-based industrial waste powder into efficient catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). pH controlled hydrothermal processing of Mo-based industrial waste powder leads to pure orthorhombic MoO 3 nanobelts (50–200 nm wide, 10 µm long) with promising OER performances at 10 mA·cm −2 with an overpotential of 324 mV and Tafel slope of 45 mV·dec −1 in alkaline electrolyte. Indeed, MoS 2 /MoO 3 nanostructures were obtained after sulfurization during hydrothermal processes of the MoO 3 nanobelts. HER tests in acidic environment show a promising overpotential of 208 mV at 10 mA·cm −2 and a Tafel slope of 94 mV·dec −1 . OER and HER performances of nanocatalysts obtained from Mo industrial waste powder are comparable or better than Mo-based nanocatalysts obtained from pure commercial Mo reagent. This work shows the great potential of reusing industrial waste for energy applications, opening a promising road to join waste management and efficient and sustainable nanocatalysts for water splitting.