Metronidazole is an effective antibacterial agent against anaerobic bacteria, and its use in the gastrointestinal field, particularly in patients with intra-abdominal infections, has increased with the availability of injectable formulations. However, a known side effect of metronidazole is central nervous system impairment, namely, metronidazole-induced encephalopathy. Here, we present a case of a 68-year-old man who underwent laparoscopic abdominoperineal resection for rectal cancer in March of year X. Postoperatively, metronidazole was administered for pelvic dead space infection and perineal wound infection, and the patient continued the oral medication after discharge. In May of year X, the patient was referred to our institution near his home for postoperative adjuvant chemotherapy. In February of year X+1, the patient developed dysarthria, and a FLAIR (fluid-attenuated inversion recovery) image on head magnetic resonance imaging revealed bilaterally symmetric hyperintense lesions in the dentate nuclei of the cerebellum. Suspecting metronidazole-induced encephalopathy based on the medication history, metronidazole was discontinued, leading to symptom improvement within approximately 3 days. We diagnosed metronidazole-induced encephalopathy based on the imaging findings and clinical observations. When using metronidazole, we should pay attention to the dosage and duration of administration and make efforts to detect side effects early.