Vallesia glabra (Vg) is a species that has been used in traditional medicine due to its secondary metabolites (alkaloids, saponins, flavonoids, phenols, and cardiac glucosides) for the treatment of measles, rheumatism, muscle aches, and eye inflammation. The biosynthesis of magnetite nanoparticles (Fe₃O₄ NPs) was carried out using an aqueous leaf extract of Vg and was characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Afterward, the magnetic adsorbent was tested for its potential to remove cationic dye from aqueous solutions at different pH and adsorbent mass and its reusability after several adsorption/desorption cycles. The XRD pattern and SEM micrographs resulted in an average size of NPs of 12.2 nm. Regarding the removal of MB from an aqueous solution, the kinetic and isotherm adsorption behavior is governed by the pseudo-second-order model and a Langmuir isotherm which describes an ionic exchange and chemisorption process between the positive partial charges of MB and Vg compounds stabilizing Fe₃O₄ NPs following a thermodynamically favorable process. Therefore, the green synthesis of NPs from Vg leaf extract is considered a sustainable alternative to removing dyes from aqueous solutions.