A 3 nm-sized cage-shaped Hg6L4 complex with large openings was quantitatively formed from trismonodentate ligand (L) and Hg2+ ions in a self-assembled manner. The cage complex shows reversible interconversion with a capsule-shaped Hg6L8 complex in response to the ratio of ligand to Hg2+. This structural switching is coupled with reversible ON/OFF switching of fluorescence between fluorescent Hg6L8 capsule and nonfluorescent Hg6L4 cage.