A thermally tunable terahertz window based on the combination of a metamaterial and the phase change material VO2 is proposed. The window is composed of two vanadium oxide films with a SiO2 layer sandwiched between them. The thermochromic phase change properties of VO2 are the key to the functionality of the window. By controlling the temperature around the room temperature of 300 K, our material can be used as a smart window and it is able to regulate both the absorption and transmission of external terahertz waves in response to changes in temperature. The absorbance can be regulated by more than 90% and the transmittance by more than 80%. The switching characteristics of the window are explained by the insulator-metal transition that vanadium oxide undergoes during the heating process, while localized surface plasmon resonance explains the perfect absorption. In addition, the designed window is not only insensitive to polarised waves, but is thermally flexible and maintains excellent performance over a wide angular range of 0° to 40°. This design will have significant potential for applications in stealth technologies, thermal sensing and switching, and terahertz energy harvesting.