Pineapple leaf fiber, a cost-effective adsorbent from agricultural biomass, was surface-modified by alkali solution (APF), followed by polyethyleneimine-grafting using carbamate-linkage reaction (APF-PEI) and used for Cr(VI) removal from aqueous solution. The surface characterizations by morphological observation, thermal behavior, FTIR and XPS indicated the successful modification of APF carbamate bonded with PEI. The adsorption was found to be strongly dependent on pH, initial Cr(VI) concentration and temperature. The adsorption of Cr(VI) onto APF and APF-PEI adsorbents could be described by the pseudo-second-order kinetic model and fitted to Langmuir adsorption. The exothermic adsorption behavior was found for APF and APF-PEI with possessing maximum adsorption capacities of 133 and 222 mg/g, respectively. Reusable performance tests indicated that APF-PEI showed good efficiency with maintaining high adsorption capacity for repeated Cr(VI) adsorptions. The novel method of PEI-modified adsorbent surfaces by generating carbamate bonds was expected to has high potential applications in adsorption systems.