Silicon has long been recognized as a beneficial element in plant biology. Recent advances in nanosilicon technology have revealed its transformative potential in legume-rhizobia symbiosis. This review synthesizes current knowledge on how silicon and SiO₂ nanoparticles (Si-NPs) influence nodulation, microbial metabolism, and soil–plant interactions. We highlight emerging evidence that Si-NPs enhance symbiotic signaling, strengthen infection pathways, and mitigate oxidative stress, thereby supporting nitrogen fixation efficiency. Beyond the rhizosphere, nanosilicon improves soil structure, microbial diversity, and plant resilience under abiotic stress, offering a multifaceted approach to sustainable agriculture. The novelty of this review lies in its integrative perspective, connecting molecular mechanisms with ecological impacts and climate-smart applications. By examining Si-NPs across three domains—soils, rhizosphere metabolites, and plants—we provide a framework for understanding their role in enhancing productivity while reducing environmental costs. Importantly, we identify critical research gaps, including the need for standardized application protocols, large-scale field validation, sustainable nanosilicon production, and robust regulatory frameworks. These insights position nanosilicon as a promising tool for advancing legume productivity, reducing reliance on synthetic fertilizers, and contributing to global food security. This review underscores silicon’s potential not only as a plant nutrient but also as a strategic agent in climate-resilient agriculture.