Inorganic–organic hybrid halide perovskite MAPbI3 (MA = CH3NH3) has attracted much attention because of its suitable band gap and strong visible-light absorption ability. However, intrinsic Pb toxicity and poor stability have hindered its further application in photovoltaic devices. Recently, Cs-based inorganic double perovskites with good stability and prominent light absorption have drawn substantial interest. Here, we have systematically investigated the electronic and optical properties of lead-free double perovskite Cs2MM′Br6 (M = Cu, Ag, and Au; M′ = Ga, In, Sb, and Bi) from first-principles calculations. Our results indicate that candidates have high stability against decomposition. The absorption coefficient of all double perovskites in the visible-light region can reach ∼105 cm–1. Direct band gaps (∼1.47 eV of Cs2AgInBr6 and ∼1.37 eV of Cs2AgGaBr6) are close to the optimal value (1.34 eV) requested by the Shockley–Queisser limit. The spectroscopic limited maximum efficiency (∼31.9% of Cs2AgInBr6 and ∼32.45% of Cs2AgGaBr6) can be obtained at the thickness of 1.5 μm. The electron mobility in Cs2AgGaBr6 is up to 160.8 cm2 V–1 s–1, similar to that of MAPbI3 (165 cm2 V–1 s–1). Our studies provide a helpful guide to designing excellent lead-free absorber layers and give a new insight into the nature of double perovskites for developing novel optoelectronic devices for renewable solar energy utilization.