Graphene and its derivatives, such as graphene oxide (GO) or reduced graphene oxide (rGO), due to their properties, have been enjoying great interest for over two decades, particularly in the context of additive manufacturing (AM) applications in recent years. High-impact polystyrene (HIPS) is a polymer used in 3D printing technology due to its high dimensional stability, low cost, and ease of processing. However, the ongoing development of AM creates the need to produce modern feedstock materials with better properties and functionality. This can be achieved by introducing reduced graphene oxide into the polymer matrix. In this study, printable composite filaments were prepared and characterized in terms of morphology and thermal and mechanical properties. Among the obtained HIPS/rGO composites, the filament containing 0.5 wt% of reduced graphene oxide had the best mechanical properties. Its tensile strength increased from 19.84 to 22.45 MPa, for pure HIPS and HIPS-0.5, respectively. Furthermore, when using the HIPS-0.5 filament in the printing process, no clogging of the nozzle was observed, which may indicate good dispersion of the rGO in the polymer matrix.