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The photodecomposition behavior of cationic porphyrin ZnTMAP⁴⁺ (zinc tetrakis-(<i>N</i>,<i>N</i>,<i>N</i>-trimethylanilinium-4-yl) porphyrin) in water and complexed with clay nanosheets was investigated by light irradiation to the Soret band of ZnTMAP⁴⁺. The decomposition of ZnTMAP⁴⁺ was observed by UV–visible absorption spectroscopy. While the decomposition quantum yield (<i>ϕ</i>_dec) was 3.4 × 10⁻⁴ in water, that was 9.4 × 10⁻⁷ on the exfoliated clay nanosheets. It was revealed that the photostability of ZnTMAP⁴⁺ was stabilized by the complex formation with clay. When ZnTMAP⁴⁺ was intercalated between the stacked clay nanosheets, <i>ϕ</i>_dec was further decreased to 4.9 × 10⁻⁷. The photostability increased by 361 times and 693 times for the exfoliated and stacked state, respectively. These results indicate that the flat clay surface has the potential to control intra- and intermolecular photochemical reactions.