The global warming event across the Kasimovian-Gzhelian boundary (KGB) of the late Pennsylvanian recorded prominent perturbations of global biogeochemical cycles, accompanied by abrupt global warming, widespread marine anoxia, and a distinct biodiversity loss. However, marine primary productivity and redox evolution during the KGB warming event have not yet been explored from the perspective of marine nitrogen cycle. Nitrogen (N) plays a key role in regulating marine primary productivity and is also sensitive to marine redox variations, which thus has the potential for tracking oceanic changes during climatic transitions. Here, we present high-resolution sedimentary delta N-15 records from three carbonate slope successions across the KGB in South China in order to investigate N cycling during the KGB warming event. Our data show consistently four-phase changes in delta N-15 from the three study sections: (I) a slow decline from similar to 12 parts per thousand to similar to 8 parts per thousand, (II) a rapid increase to 15 parts per thousand, (III) a distinct negative excursion from 15 parts per thousand to 8 parts per thousand, and (IV) constant. The delta N-15 records suggest dynamic changes in water-column denitrification and N-fixation in response to marine redox conditions. Notably, the distinct negative excursion in delta N-15 (Phase III) immediately below the KGB suggests enhanced nitrogen fixation, which possibly resulted from expanded anoxia during the KGB warming event.