Although hybrid bolted/bonded (HBB) joints possess each joint technique's benefits, the adhesive layer performance significantly affects the load-carrying capacity of hybrid joints. Nanofiber interleaving has become an efficient solution to improve the adhesion performance of bonded and HBB joints. This paper reveals the effectiveness of polystyrene (PS) nanofibers interleaving on the mechanical properties of adhesively bonded and HBB single lap joints (SLJs). For this purpose, PS nanofibers are produced via electrospinning from wasted polymers as a nature-friendly implementation. The PS nanofiber mats were interleaved between adherends as a reinforcement layer, and specimens were tested under quasi-static tensile loading. A significant improvement was seen in the peak load value of 10% for the HBB joint, and the fracture energy of the bonded joint increased by 15% with PS nanofiber modification. The failure modes of PS-reinforced specimens developed as more gradually progressive compared to neat specimens thanks to the compatibility of the recycled PS nanofiber with the epoxy and improved adhesive layer performance with PS modification. Furthermore, the morphological analyses of post-fracture specimens were monitored to realize the damage and nano-toughness mechanisms.