The present study aimed to investigate whether the addition of fentanyl to the transversus abdominis plane (TAP) block procedure may improve analgesic duration following cesarean delivery. A total of 147 nulliparous women with an American Society of Anesthesiologists physical status I-II, scheduled for elective cesarean delivery under spinal anesthesia, were enrolled in the present study. All patients underwent cesarean delivery under spinal anesthesia with 10 mg bupivacaine and 10 µg fentanyl, after which the patients underwent an ultrasound-guided bilateral TAP block with either 0.375% ropivacaine (group T_R; n=48), 0.375% ropivacaine and 50 µg subcutaneous fentanyl (group T_RS_F; n=49), or a mixture of 0.375% ropivacaine and 50 µg fentanyl (2.5 µg/ml; group T_RF; n=50) per side. The TAP block formed part of a multimodal analgesic regimen comprising patient-controlled analgesia (PCA) with intravenous fentanyl, and regular treatment with diclofenac and paracetamol. The TAP block was performed in the midaxillary line using an in-plane technique. The primary outcome was the time to the first PCA, whereas secondary outcomes were the cumulative and interval PCA consumptions, visual analogue scale (VAS) pain scores at rest and during movement, side effects assessed at 2, 6, 12, 24 and 48 h postoperatively, and patient satisfaction with postoperative analgesia. No significant differences were observed in the median time to the first PCA among the three groups (P=0.640), which were 150 min (70-720 min) in group T_R, 165 min (90-670 min) in group T_RS_F, and 190 min (70-680 min) in group T_RF. Fentanyl consumption, VAS pain scores, side effects and patient satisfaction were similar among the three groups; however, the demand for fentanyl was significantly decreased in the T_RS_F and T_RF groups at 2 h postoperatively (P=0.001 and 0.002, respectively), as compared with group T_R. No complications attributed to the TAP block were detected. In conclusion, the results of the present study suggested that the addition of 2.5 µg/ml fentanyl to the TAP block procedure was unable to improve analgesia following cesarean delivery under spinal anesthesia.