The diamond-like cubic silicon (d-Si) is widely used in modern electronics and solar cell industries. However, it is not an optimal candidate for thermoelectric application due to its high lattice thermal conductivity. Si (oP32) is a recently predicted orthorhombic silicon allotrope, whose total energy is close to that of d-Si. Using first-principles calculations and Boltzmann transport theory, we systematically investigate the thermoelectric properties of Si (oP32). The lower phonon thermal conductivity and higher power factor are obtained in Si (oP32) than those in diamond silicon. The low phonon thermal conductivity (33.77 W/mK at 300 K) is mainly due to the reduction of the phonon group velocity and enhancement of phonon–phonon scattering (including scattering phase space and strength). Meanwhile, the results also show that the thermoelectric performance along the zz lattice direction is better than that along the xx and yy lattice directions, and the figure of merit (700 K) along the zz lattice direction could approach to 2.45 and 1.75 for p-type and n-type Si (oP32), respectively. The values are much higher than those of d-Si (about 0.06)) and Si₂₄ (0.6), indicating that the Si (oP32) is a promising candidate for thermoelectric applications. Our theoretical studies shed light on the thermoelectric properties of Si (oP32) and could stimulate further experimental studies.