Recently photothermal therapy (PTT) that utilizes photothermal conversion (PTC) agents to ablate cancer under near-infrared (NIR) irradiation has attracted mounting attention owing to its excellent therapeutic efficacy and improved target selectivity. Therefore, exploring novel PTC agents with outstanding photothermal effect is one of the current research focuses. Herein, we reported a polydopamine-coated magnetic composite particle with enhanced PTC effect, which was synthesized simply through coating polydopamine (PDA) on the surface of magnetic Fe3O4 particles. Compared with magnetic Fe3O4 particles and PDA nanospheres, the core-shell nanomaterials exhibited an increased NIR absorption and thus an enhanced photothermal effect was obtained. We demonstrated the in vitro and in vivo effects of the photothermal therapy using our composite particles, and their ability as a contrast agent in the T2-weighted magnetic resonance imaging (MRI). These results indicated that the multifunctional composite particles with enhanced photothermal effect are superior to magnetic Fe3O4 particles and PDA nanospheres alone.