The present study focuses on the adsorption of organic matter mainly COD from pretreated landfill leachate of Lamdeng Khunou Solid Waste Management Plant, Manipur, India through the employment of H3PO4 treated activated carbon derived from Parkia speciosa (Petai) pods (PPAC). The adsorbent was analyzed for morphological and surface characterization by various methods including, Field emission scanning electron microscopy (FESEM), Energy Dispersive X-Ray Analysis (EDAX), Brunauer–Emmett–Teller (BET) surface area and pH at zero point charges ( pHZPC). The impacts of adsorption processes such as initial pH, temperature, equilibrium time and dose of adsorbent were considered to evaluate the performance of PPAC. At 20 °C, PPAC showed maximum COD removal of 93% within 90 min contact time, at optimum pH 2. Adsorption kinetic was able to explain by Lagergren’s pseudo-second-order equation and intraparticular diffusion models suggesting the combined behavior of both the physical and chemical adsorption of COD on PPAC. Through thermodynamics and isotherm studies, the adsorption of COD on PPAC is revealed to be exothermic with maximum monolayer coverage of 200 mg COD/g PPAC. The performance of the PPAC adsorbent is also compared with other existing reported adsorbents for treating leachate.