Description: Seaweeds provide important ecosystem services in coastal areas, and loss of these macrophytes due to anthropogenic global change and warming is a worldwide concern. Fucus vesiculosus L. (Phaeophyceae) is the most abundant and hence ecologically most important primary producer, carbon sink and habitat provider in the western Baltic Sea. Therefore, we used this keystone species to test phenotypic acclimation of physiological performance traits (growth, photosynthesis andmetabolites) of F. vesiculosus apices in a well-defined and highly resolved temperature gradient (5–29 °C), supported by highly temporally resolved measurements. Temperature requirements of growth and photosynthesis were evaluated in three weeks exposure experiments, and changing tolerance ranges for survival over timewere determined. Fucus vesiculosus was able to growand survive over a temperature range from 5 to 26 °C without any injury or visible damage of the apical growing meristem over all three weeks. However, at higherwater temperatures (≥27 °C) growth rapidly decreased fromday three onwards and progressive necrosis was observed at 28 and 29 °C. Stress-induced decrease in growth rate was already indicated by the effective quantum yield of chlorophyll fluorescence of photosystem II (PSII) several days in advance. Optimal temperature for photosynthesis (24 °C),measured as electron transport rate, was higher compared to that for growth (15–20 °C). Accordingly, the concentration ofmannitol, themain product of photosynthesis, increased with higher temperatures. Understanding physiological responses of keystone macroalgae with respect to temperature and time is important, because rising global temperatures and summer heatwave frequencies and duration may affect the ecological functions of F. vesiculosus in the western Baltic Sea.