<p>Biological nitrogen fixation is a cornerstone of terrestrial nitrogen cycling, traditionally attributed to bacterial nitrogenase activity. However, the potential contribution of rhizospheric viruses remains largely unexplored. Here, we reveal the global distribution of nitrogen-fixing genes, with widespread detection of <i>nifA</i>, <i>nifL</i>, <i>nifU</i>, and <i>nifH</i> in both bacteria and viruses, and identify <i>nifU</i> as a viral auxiliary metabolic gene (AMG). Analysis of viral communities in rhizosphere and bulk soils cultivated with cowpea showed that viral <i>nifU</i> expression was significantly upregulated in rhizosphere soils. Using ¹⁵N₂ stable-isotope tracing and virus transplantation experiments, we demonstrate that virus-encoded nitrogen-fixing AMGs, horizontally transferred from bacteria such as <i>Azospirillum thermophilum</i> (70–99% homology), increased nitrogenase activity from 1.79 to 3.14 nmol C₂H₄ g^-1 dry soil h^-1. This enhancement was accompanied by shifts in bacterial community composition, with the relative abundance of the nitrogen-fixing genus <i>Azotobacter</i> reaching 90.8%. These results uncover a previously hidden role of rhizospheric viruses in promoting bacterial nitrogen fixation, suggesting that viral-mediated gene transfer could be leveraged to enhance nitrogen cycling in soils and inform sustainable soil management strategies.</p>