For International Thermonuclear Experiment Reactor (ITER) diagnostic windows a new sealing method based on a V-shaped elastic ring has been developed. The ring, compressed by two valves, makes vacuum tight contact on the polished edge of the window material. Two types of V-shaped rings have been tested (one of silver coated copper and one in polyimide vespel SP-1) with three different window materials (fused quartz, sapphire, and ZnSe). The wavelength range of interest is from ∼0.4 to ∼10 μm. The performance of the seals to inner pressure rise resistance, the heat cycle, and acceleration at the level expected in the ITER environment has been examined. The tests have been carried out successfully for 120 mm diameter windows.