This study reports the synthesis of monosized Pr nanoparticles with a controllable size ranging from 5 to 20 nm. Pr agglomerates generated by a spark generator first size-selected by a differential mobility analyzer and subsequently sintered in-flight at different temperatures result in spherical and monocrystalline Pr nanoparticles. The dependence of size and size distribution of Pr nanoparticles has been studied as a function of deposition parameters related to spark generator, differential mobility analyzer, and sintering. Transmission electron microscopy, energy-dispersive x-ray analysis, glancing angle x-ray diffraction, and x-ray photoelectron spectroscopy studies confirm that initial Pr agglomerates and the resulting nanoparticles are metallic with d-hexagonal structure and remain stable in air during post-deposition exposure. Incomplete or partially sintered nanoparticles were found to be oxidized, resulting in the formation of amorphous oxide phase due to enhanced oxidation at grain boundaries.