Biobased and/or biodegradable plastics have been proposed as a sustainable alternative to long-lasting and fossil fuel-derived ones. Among those available, polyhydroxyalkanoate (PHA) shows great potential across a large variety of applications, but it is not used extensively because of its relatively poor physical properties. An expansion of its uses can be accomplished by developing nanocomposites where PHAs are utilized as the polymer matrix. Herein, a PHA biopolyester was melt-blended with graphene nanoplatelets (GNPs) or with a hybrid mixture of GNPs and carbon nanofibers. The resulting nanocomposites exhibited enhanced thermal stability and satisfactory mechanical properties. The hybrid nanocomposites percolated electrically at lower nanofiller loadings compared to the GNP–PHA system. The electrical conductivity at 15 wt % loading was ∼6 times higher than that of the GNP-based nanocomposite. As a result, the electromagnetic interference shielding performance of the hybrid material was around 50% better than the pure GNP-reinforced nanocomposites. The thermal conductivity increased significantly for both types of bionanocomposites and reached values in the order of 5 W K–1 m–1, with the hybrid-based material displaying once again the best performance. Considering the solvent-free and industrially compatible production method utilized to manufacture these nanocomposites, the proposed multifunctional materials can expand the range of applications of PHAs and increase the environmental sustainability of the plastic and plastic electronics industry. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsapm.0c00539. SEM high-magnification images, EMI shielding analysis, and details on the setup used to measure the thermal conductivity of the materials (PDF) This article has not yet been cited by other publications.