In this study, multiwalled carbon nanotubes were successfully modified using poly propyleneimine dendrimer. Structural characterization of the newly synthesized adsorbent confirmed the modification of nanotubes by dendrimer molecules. In order to evaluate the performance of this novel adsorbent for the removal of two direct dyes from textile wastewaters, the effect of important parameters including pH, initial dye concentration, adsorbent dosage, and inorganic salts was investigated in single and binary dye solutions. Response surface methodology was employed to find the relation between the effective parameters and dye removal efficiency. The adsorption process was optimized to reach the dye removal of 95% using response optimizer. The prepared adsorbent showed excellent adsorption behavior toward the anionic dye molecules, and the dye removal efficiencies of nanotubes were significantly improved from 17.7% to 99.9% after their modification with dendrimer. Also, the desorption tests revealed the maximum dye release of 44.01%.