Rhombohedral NASICON compounds with general formula Li_1+xTi_2-xSc_x(PO₄)₃ (0 ≤ x ≤ 0.5) have been prepared using a conventional solid-state reaction and characterized by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and impedance spectroscopy. The partial substitution of Ti⁴⁺ by Sc³⁺ and Li⁺ in pristine LiTi₂(PO₄)₃ increases unit-cell dimensions and the number of charge carriers. In Sc-rich samples, the analysis of XRD data and⁶Li/⁷Li,³¹P, and⁴⁵Sc MAS NMR spectra confirms the presence of secondary LiScO₂ and LiScP₂O₇ phases that reduce the amount of lithium incorporated in the NASICON phase. In samples with x <0.3, electrostatic repulsions between Li ions located at M1 and M3 sites increase Li mobility. For x ≥ 0.3, ionic conductivity decreases because of secondary nonconducting phases formed at grain boundaries of the NASICON particles (core-shell structures). For x = 0.2, high bulk conductivity (2.5 × 10^-3 S·cm^-1) and low activation energy (E_a = 0.25 eV) measured at room temperature make Li_1.2Ti_1.8Sc_0.2(PO₄)₃ one of the best lithium ionic conductors reported in the literature. In this material, the vacancy arrangement enhances Li conductivity.