First, magnetic nanoparticles (Fe3O4@C-ONa) were synthesized and analyzed by Fourier ‎transform infrared spectroscopy (FT-IR), X-ray diffraction spectroscopy (XRD), scanning ‎electron microscope (SEM) and vibrating sample magnetometer (VSM). These nanoparticles ‎were able to catalyze the Knoevenagel condensation ‎ between Meldrum's acid and ‎salicylaldehyde derivatives for the synthesis of coumarin-3-carboxylic acid derivatives with ‎favorable results in aqueous medium. The most suitable result is to conduct the reaction ‎under thermal conditions in the presence of Fe3O4@C-ONa (20 w%) at a temperature of ‎‎80°C and water as a solvent. In contrast, favorable results were observed in ultrasound-‎assisted conditions at a lower temperature of 40°C. After the completion of the reaction, the ‎magnetic nanoparticles were easily separated from the reaction mixture by an external ‎magnet, and after washing, they were prepared for reuse. This catalyst was used up to six ‎times with desirable catalytic activity. This protocol is a new, efficient, economical, ‎accessible, and environmentally friendly method that indicated significant advantages, ‎including a high yield of products, short reaction time, ease of preparation of the catalyst, ‎and its simple separation from the product. ‎