Background and purpose: Dyes are one of the main pollutants in the various industrial wastewaters. Therefore, the aim of this study was to assess the biosorption of acid red 18 dyes from aqueous solutions by brown macroalgae biomass “Sargassum glaucescens.” Materials and Methods: This research was a lab study. S. glaucescens was used as an adsorbent to remove acid red 18. The effect of various parameters such as pH, initial dyes concentration, adsorbent dose and equilibrium contact time were evaluated in batch adsorption. The dye concentration was measured in the wavelength of 506 nm by spectrophotometer. Results: The results showed that the equilibrium time of biosorption was 120 min. Increasing of contact time and adsorbent dose and initial dye concentration can lead to increasing of the removal efficiency. The maximum adsorption capacity of the dye was at pH: 6 respectively. It was found that the data fitted to Freundlich better than Langmuir isotherms of adsorption model. Conclusion: The S. glaucescens biomass had a satisfactory quality in dye adsorption. It can be used as an effective, inexpensive adsorbents for the dye adsorption from textile wastewater or similar industries.