Biomass of the brown seaweed Ecklonia removed both cationic Cr(III) and anionic Cr(VI). The Cr(III) was removed through an ion-exchange mechanism; the Cr(VI) was removed through a redox reaction with the biomass. Among the various pretreatments for developing an efficient biomass, thermal treatment was used in this study. After thermal treatment, the biomass characteristics were investigated using SEM, BET, FTIR, potentiometric titration, and solution analysis. The Cr(III)/Cr(VI) removal performance of the biomass was also examined. The thermal treatment altered the physical and chemical properties of the biomass. The carboxyl groups present in the biomass were decreased to 76% by thermal treatment, but the intraparticular mass transfer resistance increased. These effects of thermal treatment on the biomass reduced the adsorption rate and efficiency of Cr(III) but made the biomass stronger as a Cr(VI) reductant, resulting in an increase in the Cr(VI) reduction rate and the Cr(VI) reducing capacity of the biomass. Therefore, the thermal treatment has to be selectively adopted according to Cr(III) and Cr(VI) concentrations, since it simultaneously reduces and enhances the Cr(III) and Cr(VI) removals, respectively.