The purpose of this study was the effective safranin O removal from aqueous solution using magnetic derivative of tropical marine green calcareous alga Cymopolia barbata as a biosorbent. Cymopolia barbata biomass was magnetically modified using microwave-synthesized magnetic iron oxide nano- and microparticles; this modification was simple and inexpensive, without the need of drying step. The biosorption was studied in a batch system under various conditions. Time necessary to reach equilibrium was 90 min. The adsorption isotherms data exhibited best correlation to the Freundlich and Langmuir adsorption models. The maximum adsorption capacity reached the value 192.2 mg/g (dry mass). Kinetic data were best fitted to pseudo-second-order model. The adsorption process was exothermic and spontaneous.