The structural and the optical properties of terbium-doped yttrium oxyorthosilicate phosphor particles were analyzed. The samples were prepared through an aqueous sol–gel route and investigated in a large doping concentration range (ranging from 0.001% to 10%). The structural properties were studied by joint approach of X-ray measurements and Raman analysis, revealing the same principal structural phase (C2/c space group) independently of the Tb concentration. Radioluminescence measurements, excitation of photoluminescence, and time-resolved luminescence indicated a very efficient charge transfer mechanism from the excited level 5D3 to 5D4 level of Tb3+. A simple three-level kinetics model was applied to explain the experimental data and to give an estimation of the efficiency of the charge transfer process as a function of terbium doping. Finally, we give an estimation of the high absolute fluorescence quantum yield (0.95) for the 5D4 recombinations, almost independent of the Tb concentrations, showing that, from this level, there are no efficient nonradiative channels. The results disclose the heavy Tb-doped Y2SiO5 matrix as good candidate for the development of efficient UV light-emitting-diode-excited green phosphor in mercury-free fluorescent lamps and for a new generation of scintillator panels for X-ray radiography.