Recycling materials from waste has been considered one of the essential principles in the context of sustainable development. In this study, we used teak sawdust as the feedstock material to synthesize activated carbon (AC) samples and evaluated the application of these ACs in the adsorption of methylene blue (MB), Cd(II), and Cu(II). The sawdust was carbonized by a hydrothermal process, followed by chemical activation using K2CO3 or ZnCl2 in various weight ratios. The AC samples produced were characterized by scanning electron microscopy, Brunauer- Emmett-Teller surface area analysis, Fourier-transform infrared spectroscopy, X-ray photon spectroscopy, and mass titration of acidic groups. The characterization results showed that the ACs did possess a high surface area and rich oxygen-containing functional groups. The adsorptive amounts of MB, Cd(II), and Cu(II) on ACs approximately increased with the concentration of the activating agent: when the weight ratio of the carbonaceous material to ZnCl2 reached 1.75, the maximum adsorption capacities for MB, Cd(II), and Cu(II) were achieved, and the values were 614, 208, and 182 mg/g, respectively. The level of oxygen-containing functional groups was identified as an important factor in determining the adsorptive amounts. While the electrostatic force was the primary pathway that led to the adsorption of the tested contaminants onto the AC, the complexation reaction was a vital mechanism responsible for the adsorptive interaction between ACs and Cu(II). The high adsorption capacity of the synthetic ACs for MB, Cd(II), and Cu(II) demonstrated in this study points out the potential application of biomass-residue-based adsorbents prepared via a coupled hydrothermal carbonization/chemical activation process in wastewater treatment.