Although recent studies suggest that opioid rotation could be an effective treatment strategy for morphine-induced delirium, there have been no prospective studies to investigate the treatment effects of opioid rotation using fentanyl. The primary aim of this study was to clarify the efficacy of opioid rotation from morphine to fentanyl in symptom palliation of morphine-induced delirium. Twenty-one consecutive cancer patients with morphine-induced delirium underwent opioid rotation to fentanyl. Physicians recorded the symptom severity of delirium (the Memorial Delirium Assessment Scale, MDAS), pain, and other symptoms (categorical verbal scale from 0: none to 3: severe) and the Schedule for Team Assessment Scale (STAS) (from 0: none to 4: extreme); and performance status at the time of study enrollment and three and seven days after. Of 21 patients recruited, one patient did not complete the study. In the remaining 20 patients, morphine was substituted with transdermal fentanyl in 9 patients and parenteral fentanyl in 11 patients. Total opioid dose increased from 64 mg oral morphine equivalent/day (Day 0) to 98 mg/day (Day 7), and the median increase in total opioid dose was 42%. Treatment success, defined as an MDAS score below 10 and pain score of 2 or less, was obtained in 13 patients on Day 3 and 18 patients on Day 7. The mean MDAS score significantly decreased from 14 (Day 0) to 6.4 and 3.6 (Days 3 and 7, respectively, P < 0.001). Pain scores significantly decreased from 2.2 (Day 0) to 1.3 and 1.1 on the categorical verbal scale (Days 3 and 7, respectively, P < 0.001); from 2.6 (Day 0) to 1.6 and 1.3 on the STAS (Days 3 and 7, respectively, P < 0.001). Symptom scores of dry mouth, nausea, and vomiting significantly decreased, and performance status significantly improved. Opioid rotation from morphine to fentanyl may be effective in alleviating delirium and pain in cancer patients with morphine-induced delirium.