Antimicrobial resistance (AMR) is a growing worldwide health crisis that undermines the efficacy of traditional antibiotics against Gram-negative and Gram-positive bacteria, including multidrug-resistant bacteria like methicillin-resistant Staphylococcus aureus (MRSA). Nanotechnology presents potential solutions to AMR in the form of plant-derived nanoparticles (PDNPs), which are biocompatible, environmentally friendly to synthesize, and highly effective antibacterial agents. Underexploited plants like Bambusa spp., Amaranthus spp., and Pulicaria spp. offer a rich source of phytochemicals, such as flavonoids, alkaloids, terpenoids, and phenolics—that act as natural reducing and stabilizing agents in green nanoparticle synthesis, as well as increasing nanoparticle stability and antibacterial activity. PDNPs have their antibacterial action by several mechanisms such as bacterial membrane disruption, production of reactive oxygen species (ROS), interference with biofilm formation, and interference with DNA and protein synthesis, thus decreasing the risk of resistance. Green synthesis with underutilized plants is economical, sustainable, and eco-friendly as compared to traditional chemical and physical routes. Though promising, there are issues like inconsistency in phytochemical content, lack of extensive toxicological assessment, scalability, and regulatory hurdles that must be overcome for clinical applicability. This chapter contains an extensive review of the phytochemistry of underutilized crops, PDNP characterization, antibacterial mechanisms of PDNPs, opportunities, and challenges in the development of these nanoparticles as next-generation antibacterial agents.