Recently, the boom of graphene has aroused great interest in searching for other two-dimensional (2D) compound materials, which possess many intriguing physical and chemical properties. Interestingly, 2D allotropes of differing atomic structures show even more diverse properties. The Bi ₂Se ₃has attracted much attention due to its unique physical properties, while its allotrope has not been investigated. Based on first-principle calculations, here in this work we predict a new phase of Bi ₂Se ₃monolayer with outstanding dynamic and thermal stabilities, named as β-Bi ₂Se ₃. Notably, the β-Bi ₂Se ₃monolayer is a semiconductor with a modest direct band gap of 2.40 eV and small effective mass down to 0.52 m ₀, large absorption coefficient of 10 ⁵cm ^–1in the visible-light spectrum, suitable band edge positions for photocatalysis of water splitting. Moreover, the breaking of mirror symmetry in β-Bi ₂Se ₃along the out-of-plane direction induces vertical dipolar polarization, yielding a remarkable out-of-plane piezoelectric coefficient of 0.58 pm/V. These exceptional physical properties render the layered Bi ₂Se ₃a promising candidate for future high-speed electronics and optoelectronics.