Herein, the efficient degradation of a highly consumed antibiotic known as metronidazole (MNZ) in aqueous solutions using Co/g-C 3 N 4 /Fe 3 O 4 nanocomposite under visible light irradiation was accomplished. Initially, the photocatalyst (Co/g-C 3 N 4 /Fe 3 O 4 ) was synthesized by a simple hydrothermal method and then characterized by several analytical techniques, namely EDS, SEM, XRD, UV–vis DRS, and FTIR. The efficiency of the synthesized photocatalyst with regard to the degradation of the studied antibiotic (MNZ) under visible light irradiation was fully evaluated. The influential operational parameters affecting the efficiency of the degradation process such as pH (2–10), nanocomposite dosage (0.2–1 g/L), MNZ concentration (5–20 mg/L), and irradiation time (0–80 min) were optimized. The results revealed that the maximum degradation efficiency for MNZ was obtained under the following conditions: irradiation time of 60 min, pH = 8, MNZ concentration of 5 mg/L, and photocatalyst dosage of 0.7 g/L. In addition, the degradation of MNZ followed the pseudo-first-order kinetic model. The best rate constant ( k ) value was determined to be 0.0102 min −1 with the correlation coefficient ( R 2 ) of 0.992. According to the results of the quenching tests, it was found out that hydroxyl radicals (OH°) were the main species responsible for the MNZ degradation. Furthermore, the applied photocatalyst (Co/g-C 3 N 4 /Fe 3 O 4 ) exhibited a high level of recovery and stability after five cycles of reuse. Co/g-C 3 N 4 /Fe 3 O 4 /Vis system exhibited an excellent performance in the treatment of wastewater and real water samples. Finally, it was concluded that the synthesized nanocomposite could be potentially used as a promising and suitable photocatalyst in the degradation of other antibiotics.