The stability of monocultural, even-aged spruce forests at lower altitudes in Central Europe is seriously threatened by the prospects of global climate change. The thermostability and water use efficiency of their photosynthetic apparatus might play a vital role in their successful acclimation. In this study, photosystem II (PSII) performance (OJIP transient, rapid light curves) and thermostability were analyzed in Norway spruce (Picea abies (L.) Karst.) throughout the growing season of the exceptionally warm year 2018 (May–September) in the Western Carpathians, Slovakia. These measurements were accompanied by analysis of pigment concentrations in the needles. In addition, gas-exchange temperature curves were produced weekly from June until September to obtain intrinsic water use efficiencies. At the beginning of the growing season, needles exposed to heat stress showed significantly higher basal fluorescence and lower quantum yield, performance index, critical temperature thresholds of PSII inactivation and non-photochemical yield in comparison to other months. The overall thermostability (heat-resistance) of PSII peaked in July and August, reflected in the lowest basal fluorescence and the highest quantum yield of PSII, critical temperature thresholds and yield of non-photochemical quenching under heat stress. Additionally, the ratio between chlorophyll and carotenoids was the highest in August and had a positive impact on PSII thermostability. Moreover, the high-temperature intrinsic water use efficiency was significantly higher during July and August than in June. Results show that 15-year-old trees of Picea abies at 840 m a.s.l. exhibited acclimative seasonal responses of PSII thermostability and intrinsic water use efficiency during an exceptionally warm year. Our results suggest that mountainous P. abies at lower altitudes can acclimate their photosynthetic apparatus to higher temperatures during summer.