Nanoparticles of copper sulfide, including roxbyite Cu 1.8 S, are important materials for many applications, and they also serve as versatile templates for cation exchange reactions that transform them into derivative metal sulfide compounds and complex heterostructures. The sizes and shapes of roxbyite nanoparticles are generally determined during synthesis, and their morphologies are retained during postsynthetic modifications such as cation exchange. Here, we demonstrate postsynthetic morphological modification of roxbyite nanoparticles by treating them with 1-dodecanethiol (1-DDT) and tert-dodecanethiol (t-DDT) at temperatures ranging from 90 to 160 °C. These thiols, which are typically used as ligands and/or sulfur reagents in nanoparticle synthesis, induce morphological reshaping while maintaining composition, crystal structure, and particle volume. For example, 56 nm × 21 nm roxbyite nanorods transform to 32 nm spherical particles in the presence of 1-DDT at 130 °C for two or more hours. The nanorods progressively decrease in length and increase in width, forming a series of ellipsoids having tunable aspect ratios at intermediate time points. Control experiments point to a single-crystal-to-single-crystal pathway that involves material diffusion and migration, which can be accelerated by increasing the density of cation vacancies in the nanoparticles. Because of this pathway, the thiol-induced morphology changes are selective to the copper sulfide regions of heterostructured nanorods containing roxbyite and ZnS, Co 9 S 8 , or CuInS 2 made using partial cation exchange reactions, providing access to a library of derivative nanoparticles having otherwise inaccessible morphologies.