In this research, a nanoscale magnetic biosorbent was synthesized by incorporating magnetic nanoparticles (FeO NPs) into a natural carbon framework derived from black cumin (BC) seeds. The prepared FeO/BC was utilized as a low-cost, eco-friendly, and reusable nanobiosorbent for the removal of organic (e.g., methylene blue (MB) dye) pollutants from synthetic solutions. The results indicated that FeO/BC had extensive surface oxygenous functional groups with a high affinity for MB dye capture at different concentrations such as 10-60 mg L. The optimization results suggested the removal of 99% of methylene blue from its initial concentration (i.e., 10 mg L) using 2.0 g L of FeO/BC at pH = 7, temperature = 27 °C, and contact time = 120 min, with equilibrium adsorption capacity = 5.0 mg g and partition coefficient = ~57.0 L g. The equilibrium adsorption efficacy at the highest initial concentration (i.e., 60.0 mg L) was found to be 29.0 mg g. The adsorption isotherm was well explained by the Freundlich model for MB. The renderability of this magnetic bioadsorbent by acid treatments showed a ~66% decline in removal efficiency (%) (99% to ~33%; ~5.0 to ~1.7 mg g) for MB after six repetitive cycles of adsorption and desorption. The current FeO/BC gives a better partition coefficient than previously reported acid-washed BC seeds and other BC-seed-based nanobioadsorbents, Hence, a synthesized FeO/BC nanobiosorbent demonstrates potential for use in treating water contaminated with organic pollutants.