This study explores the potential of green-synthesized copper nanoparticles (Cu@Fv NPs) derived from fennel (Foeniculum vulgare) seeds for treating heart failure after a myocardial infarction. Fennel boasts a rich history in traditional medicine, and this research leverages its properties for a novel approach to nanomedicine. We successfully synthesized Cu@Fv NPs with well-defined spherical morphology and an average size of 48.46 nm, as confirmed by advanced characterization techniques. In vivo studies on rats with induced heart failure demonstrated the protective effects of Cu@Fv NPs. Notably, oral administration of Cu@Fv NPs at specific doses significantly reduced the lung/body weight ratio, inhibited neurohormonal activation, and led to a remarkable decrease in infarct size. Additionally, Cu@Fv NPs treatment resulted in improved cardiac function, evidenced by increased +/−dp/dt(max) and decreased LVEDP. These findings unveil the promising potential of Cu@Fv NPs as a biocompatible and potentially superior therapeutic strategy for managing heart failure post-myocardial infarction. This green synthesis method offers a sustainable and eco-friendly alternative to conventional nanoparticle production, paving the way for further exploration in nanomedicine.