Layered alkali-copper selenides: Deciphering thermoelectric properties and reaction pathways for nanostructuring β-CsCu5Se3 Copper chalcogenides offer high charge mobility and low lattice thermal conductivity but suffer from structural instability due to dynamic Cu+ migration. Here, we report a colloidal hot-injection synthesis of ternary cesium copper selenide (CsCu5Se3) nanocrystals (NCs), achieving precise control over phase, size, and morphology through tailored precursor-ligand modulation. This strategy enabled systematic exploration of stable and metastable Cs–Cu–Se phases and mechanistic investigation of nucleation and growth, providing insight into phase modulation and dimensional control at the nanoscale. CsCu5Se3 NCs exhibit low lattice thermal conductivity (∼0.5 Wm–1K–1) and an experimental zT of 0.27 at 718 K. Complementary first-principles calculations, consistent with experimental electronic and optical responses, predict a zT of 1.05 at 1000 K. These findings elucidate the formation dynamics of CsCu5Se3 and establish ABZ (A = alkali, B = metal, Z = chalcogen) NCs as tunable platforms for advanced functional applications. 11 1 481-488 481-488 American Chemical Society