In this work, the absorption of carbon dioxide (CO2) using functionalized Fe3O4 magnetic nanofluids was investigated. Nanoparticles of Fe3O4, Fe3O4-proline, Fe3O4-lysine, and Fe3O4@SiO2-NH2 were dispersed in water and methyldiethanolamine (MDEA) in order to improve the absorption performance using a high-pressure setup, in which CO2 and absorbents were in direct contact in a closed system. The effects of initial pressure of CO2, nanoparticle type, and nanoparticle concentration on absorption performance were examined. The obtained results indicated that the presence of functionalized Fe3O4 nanoparticles resulted in better results at pressure 30 bar in distilled water as a physical absorbent, and they enhanced the absorption up to 17.61 and 34.23% compared to nonfunctionalized Fe3O4 and distilled water, respectively. Subsequently, the dispersed nanoparticles in MDEA solution showed that the functionalized Fe3O4 nanoparticles were effective for CO2 separation even in chemical absorbents, and under the best conditions, they enhanced the capacity of absorption up to 16.36% at pressure 40 bar. Moreover, zeta potential analysis revealed that Fe3O4-lysine and Fe3O4@SiO2-NH2 nanoparticles are more stable than nonfunctionalized Fe3O4 and Fe3O4-proline in the base fluid.