© 2019 Hafnium oxide nanoparticles doped with trivalent samarium (HfO 2 :Sm 3+ ) were synthesized by hydrothermal route from chloride reagents. Different samarium doping concentrations (0, 0.5, 1, 3, 5 and 10 at% with respect to Hf) and different post-annealing temperatures (200, 400, 600, 800 and 1000 °C) were evaluated. The resulting nanoparticles showed an oval morphology with average sizes below 30 nm. A sensitive relationship between the samarium concentration and the resulting crystal structure of the material was observed by X-Ray diffraction and further evidenced by the cathodoluminescence and photoluminescence spectra. Low concentrations of samarium (0, 0.5 and 1 at%) generated a monoclinic phase, whereas higher concentrations of samarium (5 and 10 at%) generated a metastable cubic phase. The average crystallite sizes calculated by the Scherrer's equation were close to 8 nm and 12 nm for cubic and monoclinic phases, respectively. The luminescent emission corresponded to the characteristic 4f-4f transitions of the samarium ion with a principal peak centered at 610 nm. The best luminescent properties were obtained with the sample doped with samarium at 0.5 at%, annealed at 600 °C.