As climate change accelerates, increasing ocean temperatures and altered herbivory pressure are reshaping temperate marine ecosystems dominated by macrophyte foundation species. Despite the importance of these processes, it remains unclear how reduced above-ground biomass, due to an expected increase in herbivory (tropicalisation), will affect the thermal performance of marine macrophytes. Here, we simulated herbivory by clipping the leaf length of two temperate seagrass species, Posidonia australis and Heterozostera tasmanica, and exposed plants to a temperature gradient spanning their thermal range (6–32 °C). The effects of leaf size on thermal performance were assessed by quantifying growth, photosynthetic rates, leaf nutrient content, and pigment concentrations. Responses to clipping treatment and growing temperature were strongly species-specific. In P. australis, highly clipped individuals exhibited higher photosynthetic rates and nutrient concentrations at 32 °C relative to low-clipped and control plants, while growth did not differ between clipping treatments and survival remained constant across treatments. In contrast, clipping lowered optimal growth temperatures in H. tasmanica, and survival declined sharply at 32 °C across treatments. These findings suggest that reduced leaf size can increase thermal resilience in P. australis, whereas high herbivory in H. tasmanica is likely to increase sensitivity to thermal stress. Our results highlight that increasing herbivory pressure under climate change may therefore have positive implications for some seagrasses, emphasising the importance of considering species-specific responses when predicting the future resilience of seagrass ecosystems.