Magnetic nanocomposite, ESW-Fe3O4-Cit, was synthesized via a novel, low cost and green co-precipitation method using eggshell waste nanopowder and FeSO4. In the present procedure, eggshell (ES) nanopowder was prepared from food waste by collecting, washing, and ball milling. Nano eggshell waste (ESW) was modified by FeSO4 and trisodium citrate under natural conditions without additional alkali and a protective atmosphere. Trisodium citrate was used as a modifier and stabilizing reagent to form well dispersed small size and coated magnetic nanoparticles which should be ascribed to a large reduction of the surface electrostatic charge of the nanoparticles. The ESW-Fe3O4-Cit nanocomposite exhibits larger magnetization (14.6 emu/g) as compared to the uncoated composite (7.68 emu/g). Trisodium citrate causes an interesting increase in magnetic properties after coating on the magnetic eggshell waste nanopowder, due to the prevention of agglomeration causing self-Neel and Brownian Relaxation. The magnetic nanocomposite was fully characterized by FTIR spectroscopy, TEM and FESEM images, XRD, Zeta potential, DLS, and VSM analysis.