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The mixed-valent iridate Ba 3 InIr 2 O 9 has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits Ir 4.5+ ions in face-sharing IrO 6 octahedra forming Ir 2 O 9 dimers with three t 2g holes per dimer. Our results establish Ba 3 InIr 2 O 9 as a cluster Mott insulator. Strong intradimer hopping delocalizes the three t 2g holes in quasimolecular dimer states while interdimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of Ba 3 InIr 2 O 9 emerges from spin-orbit entangled quasimolecular moments with yet unexplored interactions, opening up a new route to unconventional magnetic properties of 5d compounds. Using single-crystal x-ray diffraction we find the monoclinic space group C2/c already at room temperature. Dielectric spectroscopy shows insulating behavior. Resonant inelastic x-ray scattering reveals a rich excitation spectrum below 1.5 eV with a sinusoidal dynamical structure factor that unambiguously demonstrates the quasimolecular character of the electronic states. Below 0.3 eV, we observe a series of excitations. According to exact diagonalization calculations, such low-energy excitations reflect the proximity of Ba 3 InIr 2 O 9 to a hopping-induced phase transition based on the condensation of a quasimolecular spin-orbit exciton. The dimer ground state roughly hosts two holes in a bonding j = 1 2 orbital and the third hole in a bonding j = 3 2 orbital.