Rubber seed shell (RSS) is a low-cost agricultural by-product. It has been utilized to produce two adsorbents, rubber seed shell charcoal (RSS-CH) and rubber seed shell activated carbon (RSS-AC) for removal of hexavalent chromium (Cr(VI)) and methylene blue (MB) from aqueous solutions. The surface morphology, thermal behavior, and FTIR were characterized to confirm the successful modification on the surface of the RSS. The effects of adsorption process such as pH, initial concentration, contact time, and temperature were also studied. The adsorption isotherm results revealed that the Langmuir isotherm model was a good fit for the Cr(VI) and MB adsorption processes. The Cr(VI) adsorption kinetic was explained by pseudo-first-order kinetic model and MB adsorption was followed with pseudo-second-order kinetic model. Moreover, the maximum adsorption capacities for Cr(VI) and MB adsorptions on the RSS-AC were 156.25 and 370.37 mg/g, respectively, which showed greater than those on the RSS-CH. Thus, the RSS-AC is a suitable, alternative, and high adsorption efficiency biosorbent for the removal of heavy metal and dye contaminants in wastewater.