In this paper, a novel magnetic responsive photonic hydrogel was prepared by embedding monodisperse Fe₃O₄ colloidal particles into network of agarose matrix, and its optical properties was investigated. First, carboxyl decorated Fe₃O₄ colloidal particles were synthesized through a hydrothermal method. Then the Fe₃O₄ colloidal particles were dispersed in water to form homogeneous colloidal solution. After mixing the colloidal solution with agarose monomer and then complete cross-linking polymerization, a uniform composite agarose hydrogel containing Fe₃O₄ particles can be obtained. With large pore size of agarose gels, the Fe₃O₄ particles in agarose matrix are still movable and colloidal crystal arrays can be achieved under external magnetic field. Moreover, the structural color of the gel can be tuned continuously by changing the strength of magnetic field. As-prepared photonic hydrogels, with dynamically tunable structural colors, provide great opportunities for applications of colloidal photonic crystals.