In this study, Sterculia foetida–based biochar (SFBC) and its chemically activated counterpart (ASFBC) were synthesized in a pyrolysis unit under nitrogen atmosphere and evaluated as economical and efficient adsorbents for removing anionic azo methyl orange and cationic triarylmethane Brilliant Green (BG) from water solutions. SFBC measured 47.04 m2/g during BET measurement and 1176.98 m2/g for ASFBC which confirms that chemical activation significantly enhanced the material surface area, porosity, and adsorption sites. The optimum removal percentages of MO and BG were 89.34% and 99.27% for SFBC and 99.90% and 99.53% for ASFBC adsorbents, which implies that ASFBC exhibited markedly higher removal percentages due to chemical activation. The equilibrium data fit best to the Langmuir model, suggesting monolayer coverage on biochar surface, having adsorption capacities of MO and BG of 104.75 and 141.34 mg/g for SFBC and 204.23 and 109.60 mg/g for ASFBC adsorbents. The kinetic data were best explained by the pseudo-second-order kinetic model. The rate constants for SFBC were 1.48 × 10−3 g/(mg·min) for MO and 1.30 × 10−3 g/(mg·min) for BG dye. For ASFBC, the rate constants were 1.52 × 10−3 g/(mg·min) for MO and 1.72 × 10−3 g/(mg·min) for BG dye. Chemisorption was the dominant adsorption mechanism. The improved performance of ASFBC is due to its higher surface area, well-developed pore structure, and oxygen-rich surface functional groups introduced during activation. SFBC and, especially, ASFBC show strong potential as low-cost adsorbents for treatment of dye-contaminated wastewater.