AbstractSuperatoms are promising as new building block materials that can be designed by precise controlling of the constituent atoms. Stannaspherene (Sn₁₂²⁻) is a rigid cage‐like cluster with icosahedral symmetry, for which one‐atom encapsulation was theoretically expected and detected in the gas phase. Here, a single‐atom introduction method into stannaspherene using a dendrimer template with polyvinylpyrrolidone (PVP) protection is demonstrated. This advanced solution‐phase synthesis allows not only the selective doping of one atom into the cluster cage, but also enable further detail characterization of optical and magnetic properties that were not possible in the gas‐phase synthesis. In other words, this liquid‐phase synthesis method has enabled the adaptation of detailed analytical methods. In this study, FeSn₁₂ was synthesized and characterized, revealing that a single Fe atom introduction in the Sn₁₂²⁻ cage result in the appearance of near‐infrared emission and enhancement in the magnetism.