To elucidate the structural features of humic acids (HAs) that potentially contribute to enhancing the activity of a tetra(p-sulfonatophenyl)porphineiron(III) (Fe(III)-TPPS) catalyst, the effects of the chemical properties of molecular weight fractionated HAs on the catalytic activity of Fe(III)-TPPS were investigated. Three fractions were obtained as the following order of molecular size: F3 < F2 < F1. The deactivation of Fe(III)-TPPS, which can be attributed to the self-degradation of Fe(III)-TPPS, was retarded in the presence of HAs, and the pseudo-first-order rate constant in the presence of F3 was the smallest of the three fractions. In addition, the highest catalytic activity, determined as the percent degradation of an organic substrate, was observed in the presence of F3. The enhanced catalytic activity of Fe(III)-TPPS was due to the formation of supramolecular complexes with HAs, and the formation constant for F3 was the largest. Thus, the F3 fraction was the most effective fraction. Solid-state CPMS C-13 NMR spectra indicated that the aromaticity of F3 was the highest of all of the fractions. Thus, it can be concluded that aromatic moieties in HAs play an important role in the formation of supramolecular complexes with Fe(III)-TPPS, leading to an enhancement in catalytic activity.