Coinage metal clusters are of great importance for a wide range of scientific fields, ranging from microscopy to catalysis. Despite their clear fundamental and technological importance, the experimental structural determination of copper clusters has attracted little attention. We fill this gap by elucidating the structure of cationic copper clusters through infrared (IR) photodissociation spectroscopy of Cu n + -Ar m complexes. Structures of Cu n + (n = 3-10) are unambiguously assigned based on the comparison of experimental IR spectra in the 70-280 cm -1 spectral range with spectra calculated using density functional theory. Whereas Cu 3 + and Cu 4 + are planar, starting from n = 5, Cu n + clusters adopt 3D structures. Each successive cluster size is composed of its predecessor with a single atom adsorbed onto the face, giving evidence of a stepwise growth.