Biological N₂ fixation is a crucial process in the marine nitrogen (N) cycle, driving primary productivity and biogeochemical cycling. As the dominant marine zooplankton, copepods play a critical role in biogeochemical fluxes and may potentially influence N₂ fixation. However, the potential interactions between copepods and diazotrophs remain poorly understood. Using samples collected during a summer 2020 cruise supported by the National Natural Science Foundation of China Shiptime Sharing Project, we combined δ¹⁵N analysis, ¹⁵N₂ tracer assays, and nifH sequencing to examine potential links between copepods and diazotrophs in the South China Sea. Diverse nifH sequences, mainly affiliated with Trichodesmium and proteobacteria, were detected in copepod samples, whereas no detectable copepod-associated N₂ fixation activity was observed. This mismatch suggests that the nifH signals likely reflected ingestion or transient retention of diazotrophs rather than active N₂ fixation within or on copepods. The close similarity between copepod-associated and seawater nifH sequences, together with positive correlations between copepod and suspended particulate δ¹⁵N and their nearshore-to-offshore gradients, supports a trophic connection between copepods and diazotroph-associated particles. These findings suggest that copepods may provide a pathway for diazotroph-derived nitrogen to enter the zooplankton food web, thereby influencing the fate of newly fixed nitrogen in oligotrophic waters.