A new modification of KOs 2 O 6 , the representative of a new structural type (Pearson symbol aP18, a=5.5668(1)Å, b=6.4519(2)Å, c=7.2356(2)Å, α=65.377(3)°, β=70.572(3)°, γ=75.613(2)° space group P−1, no. 2 was synthesized employing high pressure technique. Its structure was determined by single-crystal X-ray diffraction. The structure can be described as two OsO 6 octahedral chains relating to each other through inversion and forming big voids with K atoms inside. Quantum chemical calculations were performed on the novel compound and structurally related cubic compound. High-pressure X-ray study showed that cubic KOs 2 O 6 phase was stable up to 32.5(2)GPa at room temperature.