This work presents the green synthesis of silver nanoparticles (AgNPs) using low molecular weight lignin (LMwLg) derived from Parthenium hysterophorus as a reducing agent. Synthesis conditions were optimized by investigating the effects of silver nitrate concentration, LMwLg concentration, reaction temperature, and reaction time. The synthesized AgNPs were characterized using UV–Vis spectroscopy, FTIR, TEM, XRD, EDX, and laser diffraction analysis. Antibacterial activity was evaluated against Staphylococcus aureus, Streptococcus pyogenes, Escherichia coli, and Pseudomonas aeruginosa. The study demonstrates a sustainable approach to antibacterial nanoparticles synthesis through invasive biomass valorization and contributes to SDG 3 (Good Health and Well-being) and SDG 12 (Responsible Consumption and Production).