The paper presents the analysis of thermal conditions of coastal Lake Jamno. The long observation series (1971-2013) for the course of surface water temperature suggests its considerable warming (0.28°C∙dec-1). In the monthly cycle, an increase in surface water temperature is observed in all cases except for the winter period (December, January, February). Water was heated the fastest in April (0.58°C∙dec-1) as well as May and September (0.46°C∙dec-1). Mean annual air temperature increased at a similar rate (0.27°C∙dec-1), but the coefficient of correlation of both media equalled 0.40. This suggests that other factors could contribute to the change in surface water temperature. One of such factors could be marine water intrusions. The strongest positive correlations of water temperature in Jamno and water level in the Baltic Sea occurred in the cold half-year, and negative in the warm half-year. From 2013, on the canal connecting Lake Jamno with the Baltic Sea, a storm gate was constructed that altered many processes and phenomena occurring in the lake so far. The paper covers the natural (quasi-natural) period of functioning of the lake, and provides a point of reference for future research on its thermal regime considering among others the effect of human pressure in the form of hydrotechnical infrastructure.