The emergence of monodispersity during particle growth on a liquid substrate was investigated both by experimental methods and by computer simulation. Monodispersity arises through a novel mechanism (termed “shared coarsening”), associated with the spatial distribution of the particles; smaller particles are simultaneously consumed by several larger particles. Particle monodispersity was predicted by kinetic Monte Carlo simulation for suitable substrate adsorption probability and adatom diffusion length conditions. High particle monodispersity is predicted for low adsorption probability and low/intermediate diffusion length values. Experimentally, the formation of uniformly sized copper nanoparticles by physical vapor deposition on a liquid substrate was demonstrated. These results demonstrate, by experiment and simulation, the facile production of monodisperse particles on liquid substrates.