Polymer nanocomposites are a class of materials formed by two or more components, wherein at least one phase is dispersed in the polymer matrix at nanometric scale. The modification of nanoparticles is used in order to improve chemical compatibility between different phases and thus raise the final properties of the polymer, such as thermal, mechanical and gas barrier. With the modification of bentonite clay (BNT) using magnetite particles (MGT) it was possible to obtain nanoparticles with magnetic properties. The presence of these particles induces extrinsic magnetic properties on the clay, allowing greater control over its dispersion in the polymer matrix. After synthesis, the nanoparticles were characterized by their physico-chemical and thermal properties, indicating the formation of the magnetic clay. The nanoparticles (3 wt%) were incorporated in the poly(hydroxybutyrate) by solution and melt mixing, and the nanocomposites obtained were characterized by their physical-chemical, thermal and mechanical properties. The nanocomposites showed significant changes in these characteristics, with an improvement in the gas barrier property.