The electronic structure and related optical properties of an emerging thin-film photovoltaic material CH3NH3PbBr3 are studied. A block-shaped 伪-phase CH3NH3PbBr3 single crystal with the natural 鉄?00鉄?surface is synthesized solvothermally. The room-temperature dielectric function 蔚 = 蔚1 + i蔚2 spectrum of CH3NH3PbBr3 is determined by spectroscopic ellipsometry from 0.73 to 6.45 eV. Data are modeled with a series of Tauc鈥揕orentz oscillators, which show the absorption edge with a strong excitonic transition at 鈭?.3 eV and several above-bandgap optical structures associated with the electronic interband transitions. The energy band structure and 蔚 data of CH3NH3PbBr3 for the CH3NH3+ molecules oriented in the 鉄?11鉄?and 鉄?00鉄?directions are obtained from first-principles calculations. The overall shape of 蔚 data shows a qualitatively good agreement with experimental results. Electronic origins of major optical structures are discussed.