PtII‐catalyzed cyclization of β‐to‐β ethynylene‐bridged meso‐amino NiII porphyrin dimer 4 followed by oxidation with PbO2 afforded 1,5‐naphthyridine‐fused porphyrin dimer 5 in good yield. This dimer possesses a redox‐active 1,4‐diazabutadiene linkage that is interconvertible with its reduced 1,2‐diaminoethene linkage upon treatments with NaBH4 or PbO2. The dimer 5 exhibits an intense NIR absorption and a narrow HOMO–LUMO gap with a remarkably low reduction potential mainly due to effective bonding interactions in the LUMO through the 1,4‐diazabutadiene linkage. In contrast, the reduced dimer 7 is fairly electron‐rich with high HOMO energy and shows a relatively large HOMO–LUMO gap compared to that of 5.