The capacity and underlying mechanism of hydrochars derived from commercial D-glucose and wasted orange peels (designated as pristine-hydrochars) and further modified with nitric acid (designated as oxidized-hydrochars) to adsorb methylene blue were investigated. Both pristine- and oxidized-hydrochars were characterized by scanning electron microscopy, Brunauer–Emmet–Teller-specific surface area, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and mass titration. The maximum methylene blue adsorption capacity at 30°C estimated by the Langmuir model was found to follow the order: mGH (246 mg/g) > mOPH (107 mg/g) > OPH (59.6 mg/g) > GH (54.8 mg/g). Six adsorption mechanisms were elucidated, in which the electrostatic interaction and hydrogen bonding were identified as the primary methylene blue-hydrochar adsorptive interaction; furthermore, because the nitric acid modification process enhanced oxygen- and nitrogen-containing functional groups and unsaturated bonds on the surface of oxidized-hydrochars, the π–π and n–π interaction became minor pathways for methylene blue adsorption onto oxidized-hydrochars. Our results suggest that modified hydrochars could be used as environmentally friendly adsorbents alternative to activated carbon in dealing with methylene blue contamination in aqueous solutions.