We report the synthesis of Th1–xMnxO2 (x = 0, 0.001, 0.002, 0.004, and 0.01 wt%) nanoparticles by the urea combustion method using thorium nitrate gel followed by heat treatment at a higher temperature (T). The obtained Th1–xMnxO2 nanocrystals were characterized by x-ray diffraction (XRD), direct-current magnetization (M) measurements and electron paramagnetic resonance (EPR). XRD analysis revealed that Th1–xMnxO2 crystallizes in the cubic structure (Fm3m). M measurements showed ferromagnetic ordering at room temperature for Th0.99Mn0.01O2 samples annealed at 775 K. An intense and broad ferromagnetic resonance (FMR) having linewidth of ∼1200 G, was observed at relatively lower fields in the EPR spectra of Th0.99Mn0.01O2 samples annealed at 775 K, indicating the presence of a ferromagnetic phase at room temperature. EPR measurements were used to estimate the number of spins involved in the ferromagnetic ordering. Out of the total Mn present in Th0.99Mn0.01O2 samples, about 25% of the Mn2+ ions were found to be responsible for the ferromagnetic ordering. In addition to the FMR signal, a weak hyperfine sextet was observed at g = 2.0048 (55Mn, I = 5/2), which corresponds to the −1/2 ↔ +1/2 transition of Mn2+ ions, suggesting its presence at thorium sites (uncoupled spins). X-ray photoelectron spectra indicated that the manganese ions exist mainly as Mn2+, Mn3+, and Mn4+. The room-temperature ferromagnetism may be attributed to the coupling between these Mn2+ ions at thorium sites in ThO2 rather than due to the formation of any metastable secondary phases.