<p>Plants and plant-derived metabolites have been extensively used to fabricate multifunctional nanomaterials due to their high efficacy and biocompatibility. In this study, a green, sustainable, and cost-effective approach was employed for the synthesis of zinc oxide nanoparticles (ZnO NPs) using leaf and flower extracts of <i>Calotropis gigantea</i>, referred to as CgL-ZnO NPs and CgF-ZnO NPs, respectively. The synthesized nanoparticles were evaluated for their plant growth-promoting effects on <i>Vigna radiata</i>, <i>Vigna mungo</i>, and <i>Capsicum annuum</i>, their dye removal efficiency against Rhodamine B (RhB) in aqueous solutions, antibacterial activity against human and plant pathogens, and antioxidant properties. Additionally, the nanoparticles were assessed for compatibility with plant growth-promoting rhizobacterial strains. The results showed that highly stable, nanoscale, crystalline ZnO particles were synthesized, capped with the phytochemical constituents from <i>Calotropis gigantea</i> leaf and flower extracts. While both CgL-ZnO NPs and CgF-ZnO NPs exhibited plant growth promotion effects, the CgL-ZnO NPs demonstrated superior plant growth enhancement. Furthermore, CgL-ZnO NPs achieved the highest dye removal efficiency at 83.3%. Antibacterial tests revealed that CgL-ZnO NPs were effective against the plant pathogen <i>Rhodococcus fascians</i> and showed inhibitory effects on <i>Escherichia coli</i> and <i>Bacillus subtilis</i>. Antioxidant assays indicated a high free radical scavenging activity (RSA) for CgL-ZnO NPs. Interestingly, the synthesized nanoparticles were found to be compatible with the tested plant growth-promoting rhizobacterial strains. This research highlights the potential of <i>Calotropis gigantea</i> as a sustainable and eco-friendly source for the green synthesis of ZnO nanoparticles, with promising applications in agriculture and environmental remediation.</p> Graphical abstract <p></p>