In this study, a benzene-fused bis(thiaporphyrin) was synthesized by the retro-Diels-Alder reaction of a bicyclo[2.2.2]octadiene (BCOD)-fused bis(thiaporphyrin) precursor. This precursor was prepared by the double [3+ 1] porphyrin synthesis of the BCOD-fused dipyrrole with thiatripyrrane. The ultraviolet-visible (UV-vis) spectrum of benzene-fused bis( thiaporphyrin) in dichloromethane was extremely different from that of the corresponding benzene-fused bis(porphyrin). In contrast, extremely similar spectra were obtained in the presence of trifluoroacetic acid. The similar electronic structures in the protonated states were supported by X-ray analysis and time-dependent density-functional theory (TD-DFT) calculations.