Superparamagnetic iron oxide nanoparticles doped with zinc and modified with a cationic derivative of chitosan (SPION-Zn-CCh(+)) that exhibited enhanced activity in magnetic hyperthermia were synthesized and characterized structurally, magnetically, and biologically. The optimum zinc content and the polymer coating were found to be important to maximize the hyperthermic effect in the obtained nanoferrites, which effectively worked at concentrations as low as 0.55 mg Fe per mL. The average core diameter was found to be ca. 10 nm as revealed by SAXS and TEM measurements, while the formed small aggregates of the polymer coated nanoparticles reached a size of ca. 200 nm. The nanoferrite structure was further obtained using Mössbauer spectroscopy and XANES revealing the maximum Zn doping. The much higher (ca. 60%) intrinsic loss power determined for SPION-Zn-CCh(+) as compared to the native iron oxide nanoparticles, together with the long term (12 months) stability of their dispersion and biocompatibility, makes these nanoferrites very promising materials for magnetic hyperthermia therapy. The proposed synthetic method employing a cationic polysaccharide opens up also new opportunities for the formation of other metal ferrites with composition and structure previously unobtainable and potentially superior e.g. magnetic properties.