Because of the typical instability of copper nanoclusters, atom-precise structural elucidation of these nanoclusters has remained elusive. Herein, we report an air- and moisture-stable 23-copper nanocluster (SD/Cu23aor SD/Cu23b) isolated from the reaction of Cu(CF3COO)2, tBuC≡CH, Cu powder, and Ph2SiH2using a gradient reduction (CuII→ CuI→ Cu0) strategy (GRS), which is competent for controlling the kinetics of the reduction reaction, thus avoiding formation of pure CuIcomplexes or large Cu0nanoparticles. The solid-state structure of the Cu23nanocluster shows a rare [Cu4]0tetrahedral kernel surrounded by an outer Cu19shell, which is protected by tBuC≡C–and CF3COO–ligands. The Cu23nanocluster is a rare four-electron superatom with a 1S21P2electronic shell closure and can be crystallized in two polymorphs (R3cand R3̅) depending on the solvent used. The crystallization of SD/Cu23ain the R3cspace group is mainly governed by van der Waals forces and C–H···F interactions, whereas additional intermolecular C–H···Clchloroforminteractions are responsible for the R3̅ space group of SD/Cu23b. This work not only shows the ingenuity of a gradient reduction strategy for the synthesis of copper nanoclusters but also provides a better fundamental understanding of how to produce the polymorphic copper nanoclusters in a precisely tunable fashion.