The aimof this research was to investigate the potential of low- cost locally available stem water hyacinth (SWH) as an adsorbent for removal of acid green 20 (AG20) as an anionic dye fromaqueous solution. Equilibriumbehavior of SWH was investigated by performing batch adsorption experiments. The effect of initial dye concentration, pH, contact time and adsorbent dose on the adsorption process were evaluated. Adsorption isotherm models, Langmuir (four different forms), Freundlich and Temkin were used to simulate the equilibrium data at different experimental parameters (pH and adsorbent particle size). For Langmuir isotherm model, the four linear equations were discussed and used to obtain the isotherm parameters. Langmuir-II form was found to have the highest coefficient of determination (R2 = 0.999) compared with the other Langmuir linear equations.Also, it was found that SWH has a high adsorptive capacity towards MB dye (200 mg/g) as determined by Langmuir-II model. SWH show favorable adsorption ofAG2o dye with 0 < RL< 1. The intraparticle diffusion was also determined for the AG20-SWH system. The mechanism of the adsorption of AG20 dye onto SWH involved more than one process as indicating by the low value of Temkin constant bT (0.285 kJ/mol). Also, it was found that intraparticle diffusion was not the limiting step.