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We report an unprecedented result of self‐aggregation of [Pt(L1)Cl] (HL1=1,3‐di(5‐carboxy‐2‐pyridyl)benzene) triggered by CO2 in basic aqueous solution. The color of basic aqueous solution containing [Pt(L1)Cl] changes from yellow to blue‐green during the aggregation resulted from a reaction with CO2 in air. Upon CO2 gas bubbling, strong and broad absorption bands of aggregate assigned to the metal‐metal‐to‐ligand charge‐transfer transition appeared at 701 and 1152 nm. Recrystallization of [Pt(L1)Cl] from Na2CO3 aqueous solution afforded polymorphic crystals of red and blue‐green forms. A single X‐ray crystallography revealed that the red form of crystal consists of a Pt−Pt stacked dimer bridged by CO32− ion and one of the carboxy groups of L1 is deprotonated. An elemental analysis provided evidence that the blue‐green crystal is constructed by linear array consisting of the [Pt(L2)(CO3)]3− (HL2=1,3‐di(5‐carboxylate‐2‐pyridyl)benzene) units. The formation process of blue‐green aggregate in aqueous solution was monitored through a transient absorption spectrum, and the absorption of aggregates involved in the spectral change were examined by a global analysis. A singular value decomposition and kinetic analysis provide that there are four species resulted from the self‐assembling reaction in the solution and the maximal degree of aggregation is at least 32‐mer. [ABSTRACT FROM AUTHOR]