Magnetic defects in non-doped and iron-doped nanopowders of KTaO3 were studied by resonance and static magnetic measurements. The resonance spectra and hysteresis loops of KTaO3:Fe nanocrystalline powders were observed and investigated for the first time. Unlike bulk crystals, both non-doped and doped nanopowders exhibit two types of resonance lines, paramagnetic ones from the isolated Fe3+ ions and a ferromagnetic signal from ions involved in clusters. Theoretical description and full identification of the electron paramagnetic resonance (EPR) spectra were made. It was established that low doping (up to 0.4 mol%) increases the number of paramagnetic Fe3+ centers with axial and rhombic symmetries, and has no effect on the centers with cubic symmetry and magnetic clusters. Analysis of the temperature dependence of the spectra showed that the cluster's resonance line follows the Bloch law “T3/2”, which indicates a ferromagnetic nature of the cluster. A partial collapse of the ferromagnetic subsystem in the temperature range of 100–400 K was also found. Magnetization and other characteristics of ferromagnetic and paramagnetic subsystems were determined. An average cluster size was estimated to be equal to a length of 6.2 nm and the fractional volume of the ferromagnetic cluster system was found.