We developed a room-temperature and ultrafast Eu3+-ion doping approach for the synthesis of highly luminescent Eu-doped CaMoO4 nanoparticles. Firstly, CaMoO4 nanoparticles with a particle size of 3.9 nm are rapidly prepared using a room temperature co-precipitation approach. Subsequently, Eu-doped CaMoO4 nanoparticles with a photoluminescence quantum yield of up to 75% are synthesized by a post-cation exchange reaction at room temperature. This facile and room-temperature synthetic strategy enables us to prepare highly luminescent and extremely small rare earth ion-doped metal oxide nanocrystals.