A high-pressure phase of titanium dioxide (TiO2) with an alpha-PbO2-type structure has been identified in garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. Analytical electron microscopy indicates that this alpha-PbO2-type TiO2 occurred as an epitaxial nanometer-thick slab between twinned rutile bicrystals. Given a V-shaped curve for the equilibrium phase boundary of alpha-PbO2-type TiO2 to rutile, the stabilization pressure of alpha-PbO2-type TiO2 should be 4 to 5 gigapascals at 900 degrees to 1000 degrees C. This suggests a burial of continental crustal rocks to depths of at Least 130 kilometers. The alpha-PbO2-type TiO2 may be a useful pressure and temperature indicator in the diamond stability field.