Biosorption and bioaccumulation studies of acid orange 7 (AO7) dye by an aquatic plant, C. demersum, were investigated in the present work. The effects of different parameters such as, initial solution pH (2–11), initial dye concentration (30–90 mg/L), contact time (5–150 min), biomass dosage (1.8–23.4 g/L for living and 0.2–2.6 g/L for nonliving or dried plant) and salt concentration (0.5–20 g/L) on the decolorization of AO7 dye were investigated by batch method. The efficiency of both leaf and stem of the living plant was also assessed for the dye removal. Living plant of C. demersum exhibited higher decolorization potential as compared with the dried plant which might be attributed to the active metabolism of the former. The sorption kinetic data were successfully described with pseudo-second order model for living and dried plant. Thermodynamic studies revealed that the sorption process is endothermic in nature at studied temperature range of 283–313 K. These findings revealed that aquatic plant, C. demersum, can be a suitable alternative for decolorization of AO7 dye from aqueous solution.