We investigated the local coefficient of thermal expansion (CTE) along the diameter of a 190 nm Sn nanoparticle supported on a thin Si₃N₄ substrate. The surface and volume plasmon energies of the liquid Sn nanoparticle were measured as a function of temperature using spatially resolved valence electron energy loss spectroscopy in a scanning transmission electron microscope. We found that the slope of the volume plasmon energy versus temperature gradually increased near the surface, revealing a thermal expansion gradient in liquid nanoparticles. This effect was confined to a 3 nm-thick surface layer, where the CTE was over 1.5 times larger than that of the nanoparticle core. Meanwhile, the CTE of the core was found to be smaller than the reference value for liquid Sn, which is attributed to mechanical constraints imposed by the thin Si₃N₄ substrate. We demonstrated that temperature-induced changes in surface plasmon energy provide insights into the CTE of the outmost surface layer of Sn nanoparticles.