Water-soluble lanthanide-doped LaF3 nanocrystals were synthesized using a simple method carried out in aqueous solution at low temperature. The nanocrystals have a nearly spherical shape and an average size of below 30 nm. They consisted of well crystallized hexagonal phases. The luminescence quantum efficiency was about 16% in aqueous solution. LaF3 nanocrystals doped with different lanthanide ions (Eu3+, Ce3+, Tb3+, and Nd3+) were synthesized, which showed strong luminescence in the visible (VIS) and near-infrared (NIR) spectral regions.