Bi4TaO8Cl and Bi4TaO8Cl:Gd3+ photocatalysts were synthesized by the solvothermal technique. The phase, microstructure, optical properties, and photocatalytic activities were investigated. Both Bi4TaO8Cl and Bi4TaO8Cl:Gd3+ have the singly orthorhombic phase and the flower-like hierarchical structure. The Bi4TaO8Cl:Gd3+ has higher photocatalytic activity of MB than that of Bi4TaO8Cl under visible light irradiation. The higher photocatalytic activity of Bi4TaO8Cl:Gd3+ is induced by its lower band gap energy, the higher efficiency charge separation, and transfer process, as well as the photogenerated charge carriers with a longer time, which are confirmed by the longer absorption band edge, lower emission intensity, and higher photocurrent under visible light irradiation. The trapping experiments and electron spin resonance results suggest that O-2(-) is the predominant active species for the photocatalysis of MB by Bi4TaO8Cl and Bi4TaO8Cl:Gd3+ under visible light irradiation.