Pesticide contamination of water resources poses serious ecological and public health risks, with the pyrethroid insecticide Lambda–Cyhalothrin (LC) being of particular concern due to its toxicity, hydrophobicity, and persistence. In this study, biochar derived from the invasive weed Parthenium hysterophorus was synthesized through pyrolysis at 500 °C and subsequently activated with KOH to enhance porosity and surface functionality for efficient LC remediation. SEM, EDX, FTIR, and XRD confirmed a highly porous structure with oxygenated and aromatic surface groups, and the pH at the point of zero charge was 6.8. Batch adsorption experiments identified optimal conditions (pH 8, dose 10 g L−1, contact time 60 min), under which the biochar achieved up to 94 % LC removal at 50 mg L−1 and 88 % at 100 mg L−1. The maximum adsorption capacity was 15.7 mg g−1. Equilibrium data were best described by the Sips isotherm (qmax = 2.9647 mg g−1) and the Redlich–Peterson model (R² = 0.9998, RMSE = 0.0047), indicating heterogeneous, multilayer adsorption. Kinetics fitted a pseudo–first–order model (R² = 0.97) with signs of intraparticle diffusion. Together, the statistical metrics (R², RMSE, qmax, qe, rate constants) and characterization data indicate that adsorption is mainly physical, pore–filling, van der Waals and π–π interactions with additional hydrogen–bonding at oxygenated sites. This research demonstrates the dual benefits of using Parthenium hysterophorus for water treatment, mitigating the spread of invasive species while providing a sustainable, low–cost solution for pesticide–contaminated water treatment. The results of this study lay the foundation for future research on this adsorbent in environmental remediation applications.