Geometric structures of free iridium cluster anions, Irn-, were examined by means of ion mobility mass spectrometry and density functional theory calculation for n = 3-15 with the additional help of photoelectron spectroscopy for n = 4-10. It has been revealed that Irn- clusters with n ≥ 5 favor a square facet and take a cubic motif in contrast to the face-centered cubic structures in the corresponding nanoparticles and bulk. A growth sequence of Irn- for n = 5-15 is proposed: single Ir atoms are sequentially attached to one side of the square plane of Ir4- to form a cubic Ir8-, and are then continuously attached on one of the square facets of Ir8- for n = 9-12 and Ir12- for n = 13-15.