Biosorption is an effective technique for the removal of heavy metals from wastewater. In this study, dried, sodium hydroxide and formaldehyde modified red alga, Ceramium rubrum were used for the biosorption of copper from aqueous solution. The biosorption characteristics such as pH, biomass dosage, temperature and contact time were investigated. Also, Fourier Transform Infrared Spectrophotemetric analysis of C. rubrum was performed to identify its structure. Langmuir and Freundlich isotherm models were applied to describe the biosorption of Cu2+ onto C. rubrum biomass at the equilibrium. Langmuir model fitted well the equilibrium data for all biosorbents. Maximum biosorption capacity of dried biomass was calculated 25.51 mg/g, while it was calculated 42.92 mg/g and 30.03 mg/g for sodium hydroxide modified and formaldehyde modified biomass, respectively. Modified biomasses have higher maximum biosorption capacities indicating that modification of biomass with NaOH and HCHOH may increase responsible active sites for biosorption on the C. rubrum.