A low-cost modified soybean straw activated carbon (M-SSAC) adsorbent was prepared from soybean straw for the methylene blue (MB) adsorption from dye wastewater. The effects of pH, adsorbent dosage, initial MB concentration, and adsorption time on the adsorption of MB were investigated by response surface methodology (RSM), and the experimental conditions were optimized. The results showed that the adsorbent dosage had the most significant effect on the adsorption capacity of MB adsorbed by M-SSAC. The best adsorption effect was achieved at pH 6.6, an adsorbent dosage of 0.05 g, an initial MB concentration of 190 mg/L, and an adsorption time of 110 min, with an average adsorption capacity of 41.96 mg/g and an error of 1.19% from the predicted value. This result indicated that the optimized adsorption conditions based on the response surface method are accurate and reliable. Compared with the Freundlich model, the Langmuir model was more suitable for modeling the adsorption of MB by the M-SSAC. The adsorption process was consistent with Pseudo-second-order kinetics, and the thermodynamic calculations showed that the adsorption process was a spontaneous heat absorption process. The results suggest that M-SSAC is a promising adsorbent for effectively removing MB dye from wastewater.