The endohedral stannaspherene cluster anion [Ir@Sn12]3− was synthesized in two steps. The reaction of K4Sn9 with [IrCl(cod)]2 (cod: 1,5‐cyclooctadienyl) in ethylenediamine (en) solution first yielded the [K(2,2,2‐crypt)]+ salt (2,2,2‐crypt: 4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane) of the capped cluster anion [Sn9Ir(cod)]3−. Subsequently, crystals of this compound were dissolved in en, followed by the addition of triphenylphosphine or 1,2‐bis(diphenylphosphino)ethane and treatment at elevated temperatures. [Ir@Sn12]3− was obtained and characterized as the [K(2,2,2‐crypt)]+ salt. The isolation of [Sn9Ir(cod)]3− as an intermediate product establishes that the formation of the stannaspherene [Ir@Sn12]3− occurs through the oxidation of [Sn9Ir(cod)]3−. Among the structurally characterized tetrel cluster anions, [Ir@Sn12]3− is a unique example of a stannaspherene, and one of the rare spherical clusters encapsulating a metal atom that is not a member of Group 10. Single‐crystal structure determination shows that the novel Zintl ion cluster has nearly perfect icosahedral Ih point symmetry.