Cadmium pigments are widely used in the polymer industry. Their potential environmental risk is under debate, being the major barrier for appropriate regulation. Little is known about the leaching of hazardous cadmium ion (Cd2+) from coloured microplastics containing cadmium pigment in aquatic systems. Here, we reported the release of Cd2+ from different sized microplastics containing cadmium pigment under sunlight exposure. The release of Cd2+ was caused by the photo-dissolution of cadmium pigment induced by the reactions between photogenerated holes and the pigment lattices. The photo-dissolution process can be activated by both ultraviolet and visible light in the solar spectrum. The release kinetics is highly size-dependent. It was relatively low for microplastics with size larger than 0.85 mm but increased significantly with decreasing size for microplastics smaller than 0.85 mm. The polymer matrix was oxidized during light exposure, leading to lower average molecular weight and the formation of oxygen-containing groups. Part of the polymer matrix was degraded into soluble organic carbon under simulated sunlight, resulting in continuous Cd2+ release from the pigment particles embedded in the polymer. The polymer degradation rate is also highly size-dependent. The degradation of the polymer matrix and the release of Cd2+ were intertwined. Cadmium leaching from microplastics from a commercial product containing cadmium pigment was confirmed in water under simulated sunlight. Our results suggest that the photochemical processes of inorganic pigments will lead to the release of heavy metals from coloured plastic debris.