As climate change threatens marine ecosystems, sessile species such as kelps can be particularly impacted by rapidly changing environmental conditions. In this context, phenotypic plasticity can allow organisms to optimize their performance to cope with high temperatures. Here, we investigated the effects of thermal history on reproductive success and cross-genera- tional plasticity in the warm-temperate kelp Laminaria pallida. Our results show that reproductive success of fertile gametophytes (14oC) was improved by prior vegetative growth under warm com- pared to cold conditions. Cross-generational effects were evident; thermal tolerance of micro- scopic F1 sporophytes was modulated by the thermal history of the parental gametophytes. How- ever, the effects were complex, varying by physiological trait, exposure time and genotype (crosses). A warm thermal history increased the photosynthetic efficiency of sporophytes at mod- erate to high temperatures, while the sporophytes from a cold thermal history grew more during the first 8 d at all experimental temperatures, but this effect disappeared over time. Our results suggest that preconditioning for reproduction and cross-generational effects can play an impor- tant role in the adaptation of L. pallida to warm habitats, highlighting the importance of incorpo- rating thermal history effects in physiological studies to accurately predict the vulnerability of populations and species to future warming climates.