Awareness of plastic pollution in marine habitats, such as coral reefs, has grown in recent years. Several studies have shown that tiny particles resulting from plastic breakdown, especially microplastics, can potentially harm corals. However, to date, there is very little evidence regarding the impact that nanoplastics (<1 μm) can have on the physiology and health of corals, particularly soft corals. In this study, we exposed the soft coral Pinnigorgia flava to two concentrations (0.1 and 1 mg/L) of secondary nanoplastics-specifically nanofibers obtained from the photodegradation of polypropylene nonwoven fabrics-and their related leachates, to evaluate the coral's cellular response through the analysis of antioxidant enzyme activities (SOD, CAT, GST, GR). Chemo-physical characterization of the nano-aggregates displayed an average size of 224.3 ± 8.1 nm, while GC-MS analyses of the leachates showed a variety of mono- and dicarboxylic acids. Although both nanoplastic treatments generated a cellular oxidative stress response, the physical interaction with secondary plastic fiber nano-aggregates affected cellular homeostasis more than the chemical interaction with the released compounds, triggering a stronger antioxidant response. The activity of all antioxidant enzymes increased with higher nanofiber concentrations, while this trend was not consistently observed for the leachates. Overall, SOD and CAT were the two most responsive antioxidant enzymes in cellular detoxification. Our study highlights the significant threat that plastic nanofibers and the polymers they release may pose to coral reefs.