The geographical distribution of organisms, such as the foun- dation kelp species Saccharina latissima, is mainly driven by temperature. Globally increasing sea surface temperature and further intensification of marine heatwaves have already resulted in local extinction of kelp populations worldwide. In the present study, we investigated temporal variation in the thermal susceptibility of S. latissima by assessing stress responses of field sporophytes sampled from Helgoland (Ger- man Bight) in June 2018, August 2018 and August 2019 in heatwave scenarios. We analyzed survival, growth, maximum quantum yield of photosystem II (Fv/Fm) and pigment compo- sition. Survival decreased with increasing environmental and experimental temperatures. Growth revealed seasonal pat- terns, being higher in June than in August, whereas Fv/Fm decreased with increasing temperature, independent of the sampling time. We found an increase in the concentration of light harvesting pigments and in the de-epoxidation state of the xanthophyll cycle with higher treatment temperature. This pattern was even more pronounced at higher environmental temperature prior to the experiment (June 2018<August 2019 < August 2018). Our results show that the thermal tol- erance of S. latissima towards heatwaves in summer is signifi- cantly affected by the environmental history it previously experienced.