High-resolution absorption and laser-excited fluorescence studies of KY3F10:Er3+ core-only nanoparticles are presented. A total of 49 crystal-field energy levels, distributed amongst 13 multiplets of the Er3+ ion, have been deduced for the C4v point group symmetry centre in this material. A parametrized, single-electron crystal-field calculation provides an excellent approximation to the experimental data with optimized crystal fit parameters that are comparable to the bulk KY3F10:Er3+ crystal. Simulated spectra, based upon wavefunctions derived from the crystal-field calculations, unequivocally demonstrate that excited-state absorption is the predominant upconversion mechanism in this material – agreeing well with upconversion excitation spectra obtained for Yb3+ co-doped samples.