Climate change is acknowledged to affect directly macroalgal performance but its indirect effect through changes in algal interaction with herbivores remains poorly understood. To study effects of climate change on a macroalga–herbivore interaction, we exposed three range-margin Baltic Sea populations of Fucus (60°N 39°E, 61°N 21°E, 62°N 21°E) in September 2015 to current (15.1 °C; 5.2 PSU) and predicted future (17.5 °C; 2.6 PSU) environments for 7 months. After 6 weeks we tested the indirect effect of climate change through food quality on growth, survival and food consumption of the isopod Idotea balthica while after 7 months we tested the direct effect on algal inducible defenses. We predicted that future stressful conditions impair constitutive and inducible defense traits in Fucus thus resulting in a higher susceptibility to herbivores. The short-term climate stress did decrease thallus toughness of Fucus in one population but isopod food consumption and performance remained unchanged. After 7 months, throughout the populations, Fucus grown in future conditions was softer and more consumed. The grazing attack induced resistance in two populations but irrespective of climate change. Our results show that Fucus can cope with short-term (6 weeks) stress without indirect effects on the interactions with herbivores. However, long-term (7 months) stressed Fucus was consumed more by isopods likely due to a decrease in constitutive resistance traits rather than changes in inducible resistance. This suggests that in near future, marginal populations of Fucus might become more vulnerable to grazing losses, with possibly adverse effects on the abundance and persistence of these populations.