<p>The reinforcement of polymers with vegetal fibers is experiencing important advances in both its properties and processing as well as in its acceptance by the consumer, maintaining an exponential growth in terms of development and innovation in new composite materials. The focus of this research is given in the study of the potential effect of sargassum particles (SgP) as reinforcement in the thermomechanical properties of a polymeric matrix composed of HIPS and SBS. Three different concentrations of SgP in the matrix 5, 10, and 15 PHR (parts per hundred of additive) were evaluated and compared with a reference of virgin polymers (not modified). The evaluation of the composites in mechanical stress-elongation tests showed results of a Young's modulus higher by 58% with respect to the reference, reaching a maximum value with a content of 15 PHR of particles in the binary matrix. The composites were also evaluated in Fourier transform spectroscopy, as well as in optical microscopy.</p> Graphical abstract <p></p>