High-pressure behavior of BiGaO3 has been investigated from 0 to 20 GPa using density functional theory. It is found that BiGaO3 undergoes a pressure-induced first-order phase transition from pyroxene (Pcca) to monoclinic (Cm) at 3.5 GPa, and then to rhombohedral (R3c) at 5.2 GPa, and finally to orthorhombic (Pnma) structure at 7.4 GPa. The first phase transition (Pcca -> Cm) agrees well with the experimental results. At 5.2 GPa the possible coexistence of three ferroelectric phases, i.e., monoclinic Cm, tetragonal P4mm, and rhombohedral R3c has been predicted. The calculated values of spontaneous polarization for these phases are of 124.87, 123.48, 88.75 mu C/cm(2) for Cm, P4mm, and R3c, respectively.