This paper investigates the thermal performance of a new adaptive window which is seasonally reversible. The new window is similar in construction to the regular double-hung low-e windows, but can reverse its properties. In this window, the low-e coating faces the inside during the heating season, which reflects the infrared radiation to the inside. During the cooling season, the low-e coating faces the outside to reflect the outside heat. This window was tested in two test cells to evaluate its thermal performance. WINDOW5.2a software was used to predict the window thermal properties and BEANS building simulation software was used to predict the energy savings of the new window design. The simulation results showed an increase in the heat gain through the new window of up to 38% over the conventional double-glazing low-e window during the heating period for heavy thermal mass buildings, and 14% for light weight thermal mass buildings. At the same time, the window maintained its low heat gain properties in the cooling season . When used in moderate climates, the new adaptive window will significantly reduce energy consumption in buildings during both the heating and the cooling seasons.