Thermoelectric materials are of imperative need on account of the worldwide energy crisis. However, their efficiency is limited by the interplay of high electrical and lower thermal conductivities, that is, the figure of merit (ZT). Owing to their unique crystal structures, Cu-In-Te-based chalcogenides are suitable for both and thus have attracted much attention recently as potential thermoelectrics. Here we explore a newly developed Cu-In-Te derivative compound Cu3.52In4.16Te8. With a proper adjustment of Cu2Te doping, this material shows an ultralow lattice thermal conductivity (��L) (0.3 WK-1m-1) and, consequently, a figure of merit (ZT) as high as 1.65(��0.15) at 815 K: the highest value reported for p-type Cu-In-Te to date. The reduction in ��L is directly related to the alteration of local symmetry around the interstitial Te, resulting in an effectively optimized phonon transport through localized "rattling" of the same. Although the Hall carrier concentration reduces upon Cu2Te addition due to the unpinning of the Fermi level (EFermi) toward the conduction band minimum, the power factor remains stable. The knowledge depicted here not only demonstrates the potential of Cu3.52In4.16Te8-based alloys as a promising TE, but also provides guidelines for developing further high-performance thermoelectric materials by enhancing the electronic conductivity.