New lithium ion conductors of M-doped LiScO2 (M = Zr, Nb, and Ta) were synthesized by a solid-state reaction method. Peak shifts of the X-ray diffraction patterns revealed the formation of solid solutions with aliovalent cation doping. In addition, increase in the ionic conductivity by the M doping is indicated. The highest total conductivity of 7.94 × 10−6 S cm−1 at 623 K with an activation energy of 88 ± 5 kJ mol−1 was observed for the Zr4+ doped sample in the systems. The Zr4+ doped system showed the largest solid solution limit in Li1−xSc1−xZrxO2 (x ≈ 0.1) and continuous increase of the conductivity with increasing x. Structural analysis by Rietveld refinement indicated that the lattice expansion and lithium-ion vacancy formation by the Zr doping in the structure, which could contribute to the increase in the ionic conductivity.