We study magnetotransport in a rare-earth-doped topological insulator, Sm0.1Sb1.9Te3 single crystals, under magnetic fields up to 14 T. It is found that that the crystals exhibit Shubnikov-de Haas (SdH) oscillations in their magnetotransport behavior at low temperatures and high magnetic fields. The SdH oscillations result from the mixed contributions of bulk and surface states. We also investigate the SdH oscillations in different orientations of the magnetic field, which reveal a three-dimensional Fermi surface topology. By fitting the oscillatory resistance with the Lifshitz-Kosevich theory, we draw a Landau-level fan diagram that displays the expected nontrivial phase. In addition, the density functional theory calculations show that Sm doping changes the crystal structure and electronic structure compared with those of pure Sb2Te3. This work demonstrates that rare-earth doping is an effective way to manipulate the Fermi surface of topological insulators. Our results hold potential for the realization of exotic topological effects in magnetic topological insulators.