Co3O4 nanoparticles were size- and shape-selectively synthesized in a solvothermal approach by the thermal decomposition of either cobalt(II) acetylacetonate [Co(acac)2] or cobalt(II) nitrate [Co(NO3)2] in different solvents followed by calcination under air. Spherical, cubic, octahedral, and platelike nanoparticles with narrow size distributions and sizes ranging from 8 to 90 nm were formed in high yield. The nanoparticles were characterized by X-ray diffraction, scanning and transmission electron microscopy, energy-dispersive X-ray analysis, and X-ray photoelectron spectroscopy. The distinctive influence of the size and shape of the nanoparticles on the electrocatalytic activity in the oxygen evolution reaction is also demonstrated.