Aim: The aim of this study was to reduce the amount of diazinon from aqueous media using chemical–thermal-activated watermelon rind. Materials and Methods: This experimental study was carried out in a laboratory. First, watermelon rind was activated by chemical–thermal method. Then, the effective parameters of the diazinon adsorption process, including the initial concentration of diazinon (0.17–1 μg/L), pH (3–10), adsorbent amount (0.05–1 g/l), and contact time (30–100 min), were investigated and optimized. The amount of residual diazinon was measured by high-performance liquid chromatography. In this study, Taguchi method was used to determine the sample size and statistical analysis. Furthermore, in this study, to describe the adsorption equilibrium, Freundlich and Langmuir isotherm models were used. Results: The results showed that under an optimal pH of 6, the equilibrium time of 30 min, the amount of adsorbent 1 g/L, and the initial concentration 0.17 μg/L, the elimination efficiency of diazinon was 95.1%. Furthermore, the results of isothermic studies have shown that the removal of diazinon follows the Freundlich model (R2 = 0.921). Conclusion: Chemical–thermal-activated watermelon rind can effectively be used to remove low concentrations of diazinon from aqueous solutions.