Polycrystalline Sr2MnTiO6-delta (SMTO) nanosized powders have been prepared by the liquid mix technique. Thermogravimetric experiments show the presence of small amounts (4%) of Mn3+ in the sample. A systematic structural study shows the stabilization of nanocrystals of about 50 nm, which crystallize in an space group Pm3m, i.e. the arystotype perovskite. Microstructural analysis permits us to interpret the electrical and magnetic properties in terms of a high degree of structural disorder including intergrowths. Although the existence of polytypes in small regions is observed, the title sample can be globally (macroscopically) considered as cubic. A spin-glass type magnetic frustration behavior has been detected using different techniques: DC and AC magnetization variation with temperature and magnetic field. SMTO shows a very high dielectric constant, approaching 105 at 580 K, and being practically temperature independent above 680 K. Also, the dependence with frequency is negligible below 580 K. Electrical behavior has been interpreted considering the nanoscaled size of particles. The high semiconductive character of the sample perovskite is the key feature of the giant dielectric constant observed.