The current study highlights the synthesis of a novel biosorbent for methylene blue (MB) removal from aqueous solutions. The biosorbent was synthesized by modifying sodium alginate biopolymer (SA) with hydroxyapatite (HAp) and Fe3O4 magnetic nanoparticles (M). The surface morphology was analyzed with SEM images. XRD and BET investigated the solid’s structures, functional groups were identified by FTIR spectroscopy, and the magnetization behavior of the prepared adsorbents was investigated using VSM. The uptake of MB was investigated with respect to pH, initial dye concentration, sorbent dosage, and contact time. Studying the sorption kinetics revealed that diffusion of MB cations from the solution bulk to the solids’ surface was the rate-limiting step of the sorption. The results showed that the biosorption of methylene blue is mainly a physical process. Based on Langmuir isotherm, the maximum capacity of the SA-HAp, SA-M, and SA-M-HAp was obtained as 511.3, 538.3, and 588.2mg/g at 25 ℃, respectively. Thermodynamic studies indicate that the adsorption process was exothermic and spontaneous. The SA-M-HAp biosorbent can be regenerated up to four cycles without significant reduction (less than 5%) in the sorption efficiency toward methylene blue. This study suggests that the SA-M-HAp composite is an excellent environmentally friendly adsorbent for removing cationic dyes from aqueous solutions.