Octacalcium phosphate (OCP; Ca8(HPO4)2(PO4)4 ∙ 5H2O) is a precursor of hydroxyapatite found in human bones and teeth, and is among the inorganic substances critical for hard tissue formation and regeneration in the human body. OCP has a layered structure and can incorporate carboxylate ions into its interlayers. However, studies involving the incorporation of tetracarboxylic and multivalent (pentavalent and above) carboxylic acids into OCP have not yet been reported. In this study, we investigate the incorporation of pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid), a type of tetracarboxylic acid, into OCP. We established that pyromellitate ions could be incorporated into OCP by a wet chemical method using an acetate buffer solution containing pyromellitic acid. The derived OCP showed a brilliant blue emission under UV light owing to the incorporated pyromellitate ions. Incorporation of a carboxylic acid into OCP imparted new functions, which could enable the development of novel functional materials for biomedical applications.