In this work, we studied the methylene blue (MB) dye adsorption capacity on biochar derived from residues of Prosopis juliflora seed waste, a species found in the region of the tropical dry forest of Piojó in the Department of Atlántico, Colombia. The materials were obtained by pyrolysis at temperatures of 300, 500, and 700 °C. Biochar was characterized using Fourier transform infrared (FTIR), scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX), TGA, and Brunauer–Emmett–Teller (BET) techniques. The three biochar samples presented a macroporous, rough structure with pore size between 6 and 28 μm. The largest pore surface area observed was 1.28 m 2 /g for pyrolyzed biochar produced at 500 °C, larger than that of biochar produced at 700 °C, which was 0.83 m 2 /g. The adsorption results show that the maximum percentage of MB removal was 69%. According to SEM results, the material’s pore sizes varied on average from 6 to 28 μm. We modeled MB adsorption on biomass through three different isotherm models. The Freundlich model was the best-fitting model for the removal of MB ( K F = 1.447; 1/ n = 0.352). The kinetic results showed that the pseudo-second-order model was the best-fitting model for the sorption process ( q e = 2.94 mg/g; k 2 = 0.087 g/(mg/min –1 )). Furthermore, the recycling test showed that the biochar did not change its adsorption capacity significantly. Finally, under the experimental conditions, the thermodynamic parameters indicated that the removal of MB using biochar was an endothermic and spontaneous process; all Δ G ° values ranged from −2.14 to −0.95 kJ/mol; Δ H ° was 23.54 kJ/mol and Δ S ° was 79.5 J/mol.