This study investigates preparation of biosorbent Pleurotus eryngii immobilized on Amberlite XAD-4 and the optimal conditions for removal of Remazol Brilliant Blue R (RBBR) reactive dye from synthetic aqueous solutions. The process was optimized using the response surface methodology (RSM) developed by the application of the quadratic model associated with the central composite design. For this purpose, RSM was employed to determine the effects of operational parameters on this material as effective and available adsorbent. The investigated variables were dye initial concentration (10-60 mg L-1), solution pH (2-9), adsorbent dosage (0.1-0.5 g), and temperature (20-45 degrees C). The significant factors on each experimental design response were identified from the analysis of variance (ANOVA). The RSM indicated that optimum conditions of initial dye concentration, pH, adsorbent dosage, and temperature for maximum RBBR removal (98%) were achieved as 36.3 mg L-1, 2.0, 0.304 g, and 38.7 degrees C, respectively. The results showed that this biosorbent was an appropriate adsorbent for the removal of RBBR from aqueous solutions.