In this study, anti-leishmanial activities were performed on silver oxide nanoparticles green synthesized from hexane, ethereal, chloroform, and methanolic extracts of the Ericaria amentacea seaweed. The extracts were obtained using a soxhlet extraction system, and the silver oxide nanoparticles were synthesized through a simple and environmentally friendly method. Physicochemical characterizations, including UV spectrophotometry, transmission electron microscopy (TEM), X-ray diffraction (XRD), thermal gravimetry analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and zeta potential analysis (ZPA), were conducted to confirm the formation of silver oxide particles. The anti-leishmanial activity was evaluated in vitro using the MTT assay against the Leishmania infantum, Leishmania tropica, and Leishmania major strains. Additionally, a brine shrimp cytotoxicity test was performed on Artemia salina larvae to assess the toxicity of the products. The results showed that the anti-leishmanial activity of the synthesized silver oxide nanoparticles was significant, with inhibitory concentration values ranging from 27.16 μg/mL to 38.18 μg/mL. The lethal doses in the cytotoxicity activities were higher than 17.08 μg/mL, indicating low toxicity. These findings suggest that silver oxide nanoparticles derived from Ericaria amentacea seaweed have potential applications in the treatment of leishmaniasis. Further research is needed to elucidate the mechanisms of action and assess the in vivo efficacy of these nanoparticles. Moreover, comprehensive toxicity studies are necessary before considering their clinical use in leishmaniasis treatment.