Photodissociation of the gas-phase triiodide anion, I-3(-), was investigated using photofragment mass spectrometry. The photofragment yield (PFY) and photoproduct branching fractions were determined from the observed mass spectra in the wavelength range 270-540 nm (2.3-4.6 eV). The measurement revealed that: 1. I-3(-), photodissociation occurs with the production of I- and I-2(-), fragments in the whole energy range studied, 2. the branching ratio of I- versus I-2(-) production depends strongly on the excitation energy. These experimental findings clearly demonstrate the nonadiabatic nature of the I-3(-) photodissociation dynamics.