Giant supramolecular polyhedra are fascinating because of their large cavity, with applications in enzyme-mimetic catalysis, host-guest chemistry, and drug delivery. We report a coordination-driven, cuboctahedron-shaped, imidazolium-terpyridine-based supramolecular cage structure of about 7.3 nm diameter consisting of 12 ligands, 24 Zn²⁺ ions, and 60 or 72 anions. The steric hindrance effect from chains on the imidazolium group keeps it perpendicular to the central linkage, which readily undergoes endohedral or exohedral decoration in functionalization. A reversible pH-controlled ring closure and opening reaction of the imidazolium ligand leads to a supramolecular fusion process from a triangular prism to a cuboctahedron. The compounds were characterized unambiguously by NMR, ESI-TWIM-MS, gMS², Cryo-EM, and synchrotron X-ray diffraction techniques. Being readily modifiable through endo and exo pathways, the cages may provide a platform for the investigation of the biological mimic systems, confined nanotechnology, and confined molecular arrangements.