Nanostructured (~200 nm grain size) titanium dioxide (TiO2) ceramics were densified at temperature as low as 800 °C by pressureless sintering in a pure oxygen atmosphere. Phase transition and microstructural development of sintered samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Dielectric properties including d.c. conductivity, dielectric constant, loss tangent, and dielectric breakdown strength (BDS) were determined for samples sintered at various temperatures. The influence of sintering temperature on the microstructural development, defect chemistry, and dielectric properties of TiO2 is discussed. Nanostructured TiO2 ceramics with high sintering density (>98%) lead to improved dielectric properties; high BDS (~1800 kV/cm), low electrical conductivity (~5 × 10−15 S/cm), high dielectric constant (~130), and low loss tangent (~0.09% at 1 kHz), which is promising for application in high energy density capacitors.