Padina sp., an edible brown macroalgae found along Sri Lanka's coast, is known for its bioactive compounds, eco-friendliness, and therapeutic benefits. These compounds can reduce metal ions to form nanoparticles, acting as stabilizing and capping agents. Silver nanoparticles, renowned for their conductivity, stability, and disease-treating potential, were synthesized in this study using Padina sp. The objectives were to synthesize and characterize the nanoparticles and evaluate their antioxidant activity. Silver nanoparticles were synthesized by mixing crude methanol extract of Padina sp. with silver nitrate. Characterization was conducted using Ultraviolet–Visible (UV-Vis) spectroscopy, Dynamic Light Scattering (DLS), Zeta potential analysis, Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray (EDX) analysis, X-ray Diffraction (XRD), Fourier-Transform Infrared (FTIR) spectroscopy, and Raman spectroscopy. Antioxidant activity was assessed using the 2,2-Diphenyl-1-picrylhydrazyl (DPPH) assay. Statistical data analysis was performed using Minitab, with the results expressed as the mean ± standard deviation (SD), and paired two-sample t-tests were used to assess significant differences (p < 0.05). A visible color change from pale yellow to reddish-brown within 48 hours confirmed the formation of silver nanoparticles. UV-Vis spectroscopy showed a surface plasmon resonance peak at 420 nm, confirming the presence of nanoparticles. The DLS analysis revealed an average particle size of 73.19 nm, and the zeta potential obtained, -21.5 mV, indicated stability. The SEM images depicted spherical nanoparticles with smooth surfaces and no aggregation, while the EDX analysis confirmed 20% silver content by weight. The XRD analysis showed a face-centered cubic structure, indicating crystallinity. FTIR and Raman spectroscopy identified proteins, phenolic compounds, and amines as stabilizing agents, with polyphenolic compounds and flavonoids acting as reducing agents. In the DPPH assay, the antioxidant activity of silver nanoparticles (IC50 = 271.17 ± 3.99 μg/ml) was significantly higher than that of the crude Padina sp. extract (IC50 = 307.69 ± 9.33 μg/ml). These findings suggest that green-synthesized silver nanoparticles from Padina sp. offer a promising therapeutic strategy for diseases linked to oxidative stress.