Abstract Novel Ag3PO4 photocatalyst possesses high visible-light photocatalytic activity, but the large crystallite size and the photocorrosion severely limit its practical application. It is highly desirable to develop new Ag3PO4 photocatalytic systems with nanosized structure and good photocatalytic stability. In this work, we prepare new graphene-modified nanosized Ag3PO4 photocatalyst by in situ growth strategy in an organic solvent. The as-prepared nanosized Ag3PO4 particles–graphene composite exhibits enhanced visible-light photocatalytic activity and stability toward the degradation of methylene blue (MB) in aqueous solution compared with bare nanosized Ag3PO4 particles and conventional large-sized Ag3PO4 particles–graphene composite. This enhanced photocatalytic activity and stability arise from the positive synergetic effects of the nanosized Ag3PO4 particles and graphene sheets including an increase in the number of active adsorption sites, suppression of charge recombination, reducing the formation of Ag nanoparticles. This work shows a great potential of nanosized Ag3PO4 particles–graphene composite for environmental purification of organic pollutants.