The role of vibronic interactions in the optical properties of octasilacubane (SiH)(8) with O-h symmetry is studied. The lowest optically allowed transition occurs when an electron in the t(2u) highest occupied molecular orbital (HOMO) is promoted to the t(2g) lowest unoccupied molecular orbital (LUMO)+2 or an electron in the t(2g) HOMO-1 is promoted to the t(1u) LUMO, from the point of view of a one-electron approximation. How the frontier orbitals can couple to the molecular vibrations of (SiH)(8) is discussed in detail from density-functional-theory calculations. The vibronic coupling constants of some vibrational modes to the T-1(1u) electronic excited state that stems from the possible two lowest optically allowed transitions in (SiH)(8) are calculated. The low-frequency threefold degenerate T-2g mode of 197 cm(-1) and the twofold degenerate E-g mode of 403 cm(-1) have large vibronic coupling constants, and the vibronic coupling would provide the absorption band at 3.5 eV with approximate energy separations of 25 and 50 meV, respectively, if the optical absorption spectrum is measured in the gas phase or dilute matrix.