This report describes the first synthesis of a highly crystalline YAG:Tb nanophosphor inside the pores of a mesoporous silica monolith (MSM). A simple wet impregnation procedure using a YAG:Tb sol and a MSM platform prepared by a sol–gel method was adopted to prepare highly homogeneous MSM–YAG:Tb luminescent composites. The morphological, structural and luminescence properties of in situ generated YAG:Tb nanocrystals, investigated using XRD, TEM, HRTEM, EDS, FTIR, nitrogen gas sorption and photoluminescence measurements, were compared to those of the bulk YAG:Tb phosphor. The specific surface area and the pore volume of the MSM host material were found to be lower when YAG:Tb nanoparticles were grown in the pores of the MSM material. The particle diameter of the nano-sized YAG:Tb phosphor was estimated to be about 23 nm. The MSM–YAG:Tb composite exhibited a strong green fluorescence emission with the characteristic main emission band of Tb3+ located at 548 nm. The prepared composite also showed a shorter photoluminescence (PL) lifetime and excitation bands shifted to lower wavelengths compared to the neat YAG:Tb phosphor. The as-prepared MSM–YAG:Tb material could be integrated into optoelectronic devices and holds good potential to be applied in decorative lighting