Easy preparation of well-dispersed inorganic nanoparticles is critical for their practical applications. By using jellyfish mesoglea, a biological hydrogel mainly composed of nanofibers, as a template, nanocomposite hydrogels with well-dispersed Fe₃O₄ nanoparticles are successfully synthesized through the co-precipitation method. Fe₃O₄ nanoparticles, mostly less than 20 nm, are uniformly formed and distributed on the nanofibers. With the increase of Fe²⁺/Fe³⁺ concentration, more nanoparticles are formed and hence the size of the nanofibers increases, and only very less significant aggregation of Fe₃O₄ nanoparticles is observed at a high Fe²⁺/Fe³⁺ concentration of 1 M. The JF/Fe₃O₄ nanocomposite hydrogels have very high Fe₃O₄ content, which can achieve about 79%. The nanocomposite hydrogels display nearly superparamagnetism, and show smaller saturation magnetizations (M_s) than that of bulk Fe₃O₄. Moreover, introduction of Fe₃O₄ nanoparticles increases the BET surface areas of the gels. The JF/Fe₃O₄ nanocomposite hydrogels exhibit high activity in catalyzing the oxidative degradation of xylenol orange and excellent reusability.