Lanthanide ions are widely used as dopants for halide perovskites for their broad energy level coverage from the visible to near-infrared (NIR) range. In this work, Cs 2 NaScCl 6 :Er 3+ was synthesized by an improved solid-state reaction method, which showed effective NIR emission under ultraviolet excitation. Through calculations based on density functional theory and Bader charge analysis, it is shown that [ErCl 6 ] 3- octahedra show a strong localization effect in the Cs 2 NaScCl 6 :Er 3+ lattice, which is conducive to the charge transfer process of Cl-Er 3+ , and charge transfer sensitization is responsible for the efficient visible to NIR luminescence of Er 3+ , where the NIR emission around λ em = 1540 nm originated from the Er 3+ : 4 I 13/2 → 4 I 15/2 transition with an ultrahigh photoluminescence quantum yield that reached ∼28.3%. Notably, Cs 2 NaScCl 6 :Er 3+ also exhibited bright upconversion luminescence of green light (at 540 nm) under excitation by a variety of NIR laser diodes (808, 980, and 1550 nm) via self-sensitization processes.