ReO3 nanoparticles in the diameter range of 8.5-32.5 nm have been prepared by decomposition of the Re2O7-dioxane complex under solvothermal conditions and characterized by X-ray diffraction, electron microscopy, optical spectroscopy, and scanning probe microscopy. The nanoparticles have the cubic (Pm3m {221} space group) structure with the lattice parameter increasing with decreasing size. The particles are metallic and show a plasmon band around 520 nm, which becomes blue-shifted with a decrease in size. The metallicity of the nanoparticles is also confirmed by tunneling conductance measurements. The nanoparticles show paramagnetic or diamagnetic behavior depending on the size, with evidence for superparamagnetism at low temperatures when the size is small.