Herein, the first preparation of a novel 3D hierarchically porous carbon (3D HPC) via precarbonization and in situ alkali activation, wherein mesoporous polydivinylbenzene (meso-PDVB) synthesized by a facile and general solvothermal route was used as the carbon precursor, is reported. Its physico-chemical properties were characterized by SEM, TEM, XRD, FT-IR, elemental analysis, Raman spectroscopy and N₂ adsorption–desorption isotherm. 3D HPC exhibited a very large SSA of 3131.32 m² g⁻¹ and a pore volume of 1.462 cm³ g⁻¹. Moreover, 3D HPC was used to eliminate a dye from wastewater for the first time. The effects of the contact time, initial dye concentration, temperature and solution pH on the adsorption of methylene blue (MB) onto 3D HPC were investigated by batch techniques. The isotherm and kinetics data were well described by a Langmuir model and a pseudo-second-order kinetic model, respectively. 3D HPC had a high adsorption affinity for MB over a broad pH range. 3D HPC displayed remarkably strong adsorption of MB, with a maximum adsorption capacity of 717.77 mg g⁻¹ at 308 K. The adsorption process was endothermic and spontaneous, and the kinetics was controlled by film and intra-particle diffusion. The mechanism of the adsorption of MB was mainly attributed to van der Waals forces, π–π stacking and electrostatic interactions. In addition, 3D HPC displayed excellent reusability and exhibited promising potential for the treatment of dyes in wastewater.