We first report the hydrogen storage properties of Rh nanoparticles having a diameter over 10 nm and the change in the temperature dependences of hydrogen storage based on particle sizes of Rh nanoparticles. Hydrogen storage was investigated by hydrogen pressure composition isotherm and solid-state (HNMR)-H-2 measurements. We demonstrate that Rh nanoparticles with dimensions of ca. 10 nm absorb hydrogen similar to smaller Rh nanoparticles; however, the enthalpy of hydrogen storage in Rh nanoparticles is changed from exothermic to endothermic with increasing particle size, with a critical size between 7.1 and 10.5 nm. The thermodynamics of hydrogen storage in Rh can be tuned by controlling the particle size.