A biosorbent was prepared by drying garlic (Allium sativum L.) under vacuum and tested in its powder form for its mercury adsorption capability in aqueous solution. Results show that garlic powder has a good adsorption capacity for mercury and that the mercury concentration of the solution has a significant impact on the adsorption capacity of the biosorbent. The Langmuir and Freundlich adsorption isotherms were also constructed. The adsorption capacity, qmax, adsorption efficiency, b, and correclation confficient, r2 for the Langmuir model were 0.6497, 0.4903, and 0.9980, respectively. For the Freundlich model, the model parameters, KF, 1/n, and r2 for mercury were 4.1879, 0.3467, and 0.9518, respectively. Langmuir adsorption isotherm was better suited for the adsorption of mercury onto garlic powder, and that the maximum amount of mercury adsorbed (qmax) was 0.6497 mg/g.