Garlic mustard (Alliaria petiolata) is an invasive Eurasian species that is now widespread in North America. Like some other invasive species, garlic mustard is known to exude biochemical compounds that can reduce the fitness of native species when it invades a new habitat. Compounds leached from garlic mustard can reduce growth and survival of mycorrhizal fungi associated with forest trees in eastern North America. We tested whether these compounds could also inhibit the growth or survival of fungi known to infect arthropods, the so-called entomopathogenic (EM) fungi. We found that growth of Beauveria bassiana (Bb), a widespread EM fungus, was significantly reduced when spores were incubated on agar plates made with leachate from garlic mustard. When leachate was added to soil that had been inoculated with Bb spores, waxworms were significantly less infected with Bb than they were on soil inoculated with Bb spores alone. Finally, waxworms were less infected with EM spores when they were held in soil collected from field plots with abundant garlic mustard plants compared to neighbouring soils without garlic mustard plants. Together, these results demonstrate the potential for garlic mustard leachate to significantly inhibit the growth of EM fungi and thereby suppress the effects of EM fungi on arthropods. This effect of garlic mustard could be beneficial to humans if it reduces mortality of arthropods that provide ecosystem services, but it could also be harmful to humans if it reduces mortality of pest insects or arthropod vectors of disease, such as ixodid ticks.