Patients with cancer and comorbid psychiatric disorders, including schizophrenia, may experience aggravation of psychiatric symptoms during cancer treatment. Oncology hospitals without dedicated psychiatric wards sometimes face challenges in managing cases with severe psychiatric symptoms. Concerns exist that the worsening of psychiatric symptoms may lead to the interruption of cancer treatment. A woman in her 60s had chronic schizophrenia, with an estimated onset in her 20s. She had long been prescribed quetiapine (150 mg/day, oral), haloperidol (0.75 mg/day, oral), nitrazepam (10 mg/day, oral), and paroxetine (20 mg/day, oral). At X years, she was diagnosed with ovarian cancer and underwent surgery following discontinuation of the psychotropics. On day 2 after hospitalization, she became confused, hallucinatory, and delusional with severe agitation. Although initially stabilized, the patient exhibited worsening psychiatric symptoms again on day 7. Ultimately, the patient was mentally stabilized with adjusted doses of oral quetiapine (250 mg/day) and haloperidol (1.5 mg/day, oral) and was discharged on day 16. Thanks to the intervention, cancer treatment continued uninterrupted. Patients with comorbid schizophrenia and cancer may present severe psychiatric symptoms in the cancer perioperative period, even if schizophrenia is in a chronic phase. Postoperative delirium, withdrawal delirium, and exacerbation of schizophrenia were speculated to be the possible contributing factors in this case. When patients with cancer also have schizophrenia, oncologists and liaison psychiatrists need to carefully monitor their mental status to prevent interruptions in cancer treatment.