The search for new methods of synthesising magnetic nanoparticles is crucial because the properties of these materials are extremely sensitive to the way they are obtained. Innovative synthesis techniques allow these parameters to be precisely controlled, which translates into the ability to design nanoparticles with precisely tailored functions. This is important for both technological and biomedical applications. New methods can also increase production efficiency and repeatability, reduce the toxicity of reagents, and enable the production of nanomaterials with previously unavailable properties. This study attempts to synthesise chitosan-functionalised magnetite nanoparticles in an innovative way using ultrasound. The aim of the study was to compare the structural, magnetic and calorimetric properties of nanoparticles synthesised by co-precipitation and by a new modified method using ultrasound. In addition, the effect of the amount of chitosan on the physical properties of the nanoparticles was examined. The structure and morphology of the obtained nanoparticles were characterized by X-ray diffraction and transmission electron microscopy, determining their average sizes and size distributions. The effect of the amount of surfactant and the method of nanocrystal synthesis on the magnetic properties was analysed using Mössbauer spectroscopy, while calorimetric measurements demonstrated the suitability of magnetite nanoparticles with a chitosan-modified surface for potential hyperthermia applications. Mössbauer studies have shown that an increased amount of chitosan weakens the interactions between nanoparticles, facilitating their transition to a superparamagnetic state. Samples synthesised using the new method exhibit superparamagnetic properties at lower temperatures than samples modified by co-precipitation. Calorimetric measurements indicate better heating properties in samples obtained using ultrasound.