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Polyradical cages are of great interest because they show very fascinating physical and chemical properties, but many challenges remain, especially for their synthesis and characterization. Herein, we present the synthesis of a polyradical cation cage 14.+ through post‐synthetic oxidation of a redox‐active phenothiazine‐based Pd2L4‐type coordination cage 1. It′s worth noting that 1 exhibits excellent reversible electrochemical and chemical redox activity due to the introduction of a bulky 3,5‐di‐tert‐butyl‐4‐methoxyphenyl substituent. The generation of 14.+ through reversible electrochemical oxidation is investigated by in situ UV/Vis‐NIR and EPR spectroelectrochemistry. Meanwhile, chemical oxidation of 1 can also produce 14.+ which can be reversibly reduced back to the original cage 1, and the process is monitored by EPR and NMR spectroscopies. Eventually, we succeed in the isolation and single crystal X‐ray diffraction analysis of 14.+, whose electronic structure and conformation are distinct to original 1. The magnetic susceptibility measurements indicate the predominantly antiferromagnetic interactions between the four phenothiazine radical cations in 14.+. We believe that our study including the facile synthesis methodology and in situ spectroelectrochemistry will shed some light on the synthesis and characterization of novel polyradical systems, opening more perspectives for developing functional supramolecular cages. [ABSTRACT FROM AUTHOR]