The humic acid-modified expanded perlite (HA/EP) was first reported and developed as a low-cost adsorbent for removal of cationic dye methylene blue (MB). Effects of pH, ionic strength, adsorption time, adsorption temperature, and the initial concentration of MB on removal efficiency were investigated. Under optimized conditions, the experimental maximum adsorption capacity of HA/EP could achieve 78.2 mg/g, which was significantly higher than that of EP (< 1 mg/g). Adsorption kinetics, equilibrium, and adsorption thermodynamics were also evaluated. Both experimental and theoretical results demonstrate that the HA/EP can be used as a promising adsorbent for removal of cationic dyes from aqueous solution.