Marine biofouling-the colonization of small marine microorganisms on surfaces that are directly exposed to seawater, such as ships' hulls-is an expensive problem that is currently without an environmentally compatible solution(1). Biofouling leads to increased hydrodynamic drag, which, in turn, causes increased fuel consumption and greenhouse gas emissions. Tributyltin-free antifouling coatings and paints(1-4) based on metal complexes or biocides have been shown to efficiently prevent marine biofouling. However, these materials can damage(5) the environment through metal leaching (for example, of copper and zinc)(6) and bacteria resistance(7). Here, we show that vanadium pentoxide nanowires act like naturally occurring vanadium haloperoxidases(8) to prevent marine biofouling. In the presence of bromide ions and hydrogen peroxide, the nanowires catalyse the oxidation of bromide ions to hypobromous acid (HOBr). Singlet molecular oxygen (O-1(2)) is formed and this exerts strong antibacterial activity, which prevents marine biofouling without being toxic to marine biota. Vanadium pentoxide nanowires have the potential to be an alternative approach to conventional anti-biofouling agents.