Aromatic ring fusion to porphyrins and their derivatives represents an attractive route to tune the molecular conjugation and thus expand their functionalities. Here, we report the expansion of the aromatic π-system of palladium tetraphenyltetrabenzoporphyrins (Pd-TPTBP) viasurface-assisted γ-orthocyclodehydrogenation on Ag(111). The chemical transformation of Pd-TPTBP into different products at an elevated temperature of 600 K was revealed at the single-molecule level using bond-resolved scanning tunneling microscopy with a CO-functionalized tip. We captured a series of γ-orthocyclodehydrogenation products, wherein the maximum extent to which the reaction can progress is associated with 7-fold C–C formation to afford nearly planar γ-orthofused porphyrins with 66 conjugated π-electrons. In addition, a small number of molecules undergo C–C bond dissociation of meso-phenyl at elevated temperature, producing fully planar γ-orthofused products lacking one or two phenyl moieties. Scanning tunneling spectroscopy measurements and DFT calculations suggest the electronic gap of the γ-orthofused porphyrin decreases compared to that of the precursor. The HOMO and LUMO of the planar γ-orthofused products are localized on the partially fused benzo moieties and the meso-position, respectively.