•3D Ag2O–Ag/TiO2 composites have been synthesized through a facile method.•3D Ag2O–Ag/TiO2 composites exhibit large photocatalytic adsorption capacity, high selectivity, and excellent trace removal performance of I− under visible light.•3D Ag2O–Ag/TiO2 composites could be easily separated and regenerated.•The adsorption capacity of Ag2O for I− is enlarged 4.4 times by the photooxidation of Ag/TiO2.•The cooperative effects mechanism between Ag2O and Ag/TiO2 is proposed and verified. Three-dimensional Ag2O and Ag co-loaded TiO2 (3D Ag2O–Ag/TiO2) composites have been synthesized through a facile method, characterized using SEM, EDX, TEM, XRD, XPS, UV–vis DRS, BET techniques, and applied to remove radioactive iodide ions (I−). The photocatalytic adsorption capacity (207.6mg/g) of the 3D Ag2O–Ag/TiO2 spheres under visible light is four times higher than that in the dark, which is barely affected by other ions, even in simulated salt lake water where the concentration of Cl− is up to 590 times that of I−. The capability of the composites to remove even trace amounts of I− from different types of water, e.g., deionized or salt lake water, is demonstrated. The composites also feature good reusability, as they were separated after photocatalytic adsorption and still performed well after a simple regeneration. Furthermore, a mechanism explaining the highly efficient removal of radioactive I− has been proposed according to characterization analyses of the composites after adsorption and subsequently been verified by adsorption and desorption experiments. The proposed cooperative effects mechanism considers the interplay of three different phenomena, namely, the adsorption performance of Ag2O for I−, the photocatalytic ability of Ag/TiO2 for oxidation of I−, and the readsorption performance of AgI for I2.