Norway spruce (Picea abies (L.) Karst) populations are facing increased biotic and abiotic pressure especially at lower range of their natural distribution. Elevated summer temperatures induced by global climate change might impair the physiological vitality of Norway spruce forests causing overall decrease of their tolerance to drought periods as well as increased risk from other disturbances. In this study, we analysed the ability of Norway spruce to acclimate to higher temperatures during the summer by improving the thermostability of its photosynthetic apparatus. We utilized short-term heat stress simulation with water baths followed by fast and slow kinetics of chlorophyll a fluorescence. Measurements were conducted once a month from May until September. Simultaneously, needles were sampled for pigments concentration analysis and gas exchange measurements were conducted on the same individuals. We found that Norway spruce is able to improve its PSII thermostability during summer with maximal performance after short-term heat stress occurring in July and August. This acclimation response was positively correlated with chlorophylls and carotenoids ratio which significantly differed between the observed months. Moreover, there was no significant difference in assimilation rate between during the experiment. Our results suggest that healthy trees of Norway spruce at a lower range of distribution can acclimate to higher temperatures during summer and maintain a high assimilation rate throughout vegetation season, by improving PSII thermostability via pigments ratio adjustment.