By measuring the electrical resistivity in TPP[Fe^III(tbp)(CN)₂]₂ (TPP = tetraphenylphosphonium and tbp = tetrabenzoporphyrin) under the application of a static magnetic field, a giant negative magnetoresistance (MR) effect with high anisotropy is observed. More specifically, the MR ratio at 13 K under a field of 9 T perpendicular to the c axis is −70%, whereas the MR ratio under a field parallel to the c axis is −40%. Furthermore, electron spin resonance (ESR) measurements indicate large anisotropy in the principal g-values of d spin (S = 1/2) in the [Fe^III(tbp)(CN)₂] unit; the g₁ value almost perpendicular to the tbp plane and the g₂ and g₃ values almost parallel to the tbp plane are 3.60, 1.24, and 0.39, respectively. It is revealed that the anisotropy in the MR effect arises from the anisotropy in the d spin, suggesting that the d spins in TPP[Fe^III(tbp)(CN)₂]₂ affect the π-conduction electron via the intramolecular π–d interaction. The anisotropy and magnitude in the giant negative MR effect for TPP[Fe^III(tbp)(CN)₂]₂ are smaller than the corresponding values for the isostructural phthalocyanine (Pc) analogue TPP[Fe^III(Pc)(CN)₂]₂. This is consistent with the fact that the intermolecular antiferromagnetic d–d interaction in TPP[Fe^III(tbp)(CN)₂]₂ (suggested by the Weiss temperature: Θ = −8.0 K) is weaker than that in TPP[Fe^III(Pc)(CN)₂]₂ (Θ = −12.3 K). This indicates that the minor modification in coordination complexes can significantly affect the MR effect via tuning the intermolecular d–d interaction as well as the intermolecular π–π overlap.