This study presents the variation in the accumulated heat content of the alpine-type lake Morskie Oko in the highest mountain range of the Carpathians – the Tatras. The analysis was based on detailed water temperature measurements carried out in 2007 at the deepest site of the lake. Additionally, the changeability of heat content in 1973-2012 was assessed using measurements performed at one site near the shore at a depth of 0.4 m. The highest and the lowest values of accumulated heat content were recorded in August and February, which is typical for lakes in intermediate latitudes. The monthly average accumulated heat content in the studied year amounted to 186,830 GJ. Long-term dynamics of heat content indicated an upward trend, particularly from the late 1990s, which was closely connected with an increase in air temperature. This investigation indicated that mountain lakes – commonly regarded as cool – may be a potential source of “clean” energy in hard-to-reach terrain (e.g., for the needs of mountain refuges, holiday camps, etc.).