Overfishing herbivorous fishes can weaken herbivory pressure on coral reefs, driving or maintaining shifts from coral to algal dominance in these imperiled ecosystems. Traditionally, quantifying absolute changes in herbivory pressure requires long-term data and/or population models that can be expensive and time consuming. However, trait-based ecology, a common approach in terrestrial ecology that is rarely used in marine ecosystems, could provide a simple and rapid approach for estimating herbivory on tropical reefs. We tested this hypothesis on five fringing reefs on Moorea, French Polynesia. We estimated relative herbivory with a bioassay and measured seven algal traits related to herbivory (volume, height, wet and dry weight, percent calcification, tensile strength, and toughness) of the calcifying macroalgae, Halimeda opuntia. Herbivory differed significantly among sites (ANOVA p <0.0001), with an ~ 8-fold difference between the lowest and highest pressures. Algal traits varied concomitantly; algae from high herbivory sites were larger, allocated more mass to structure, and were more functionally diverse (PERMANOVA, p < 0.0001) than low herbivory sites. A field experiment manipulating herbivory (none, ambient, simulated) showed algae subject to simulated and ambient herbivory occupied similar trait space, confirming that herbivory caused algal trait differences. Results show the power of trait-based ecology for estimating herbivory on macroalgal-dominated coral reefs, providing an important tool for the study and management of coral reef ecosystems.