Seaweed-herbivore interactions are often mediated by environmental conditions, yet the roles of emerging anthropogenic stressors on these interactions are poorly understood. For example, chemical contaminants have unknown consequences on seaweed inducible resistance and herbivore response to these defenses despite known deleterious effects of contaminants on animal inducible defenses. Here, we investigated the effect of copper contamination on the effectiveness of inducible resistance in the brown seaweed, Silvetia compressa, to a snail herbivore, Tegula funebralis. We examined the effectiveness of inducible resistance for organisms exposed to copper at two time points, either during induction or after herbivores had already induced seaweed. Under ambient conditions, non-grazed tissues were more palatable than grazed tissues. However, copper additions negated the preference for non-grazed tissues regardless of the timing of copper exposure, suggesting that copper decreased the effectiveness of inducible defense. This effect of copper was only evident during five day exposures, and was largely a result of copper influencing snail behavior. Thus, contaminants can stress communities by reducing the effectiveness of seaweed inducible defenses. Given the ubiquity of seaweed inducible resistance and their potential influence on herbivore performance and populations, we hypothesize that reduced effectiveness of seaweed inducible defenses in the presence of copper contamination can have cascading effects on marine communities.