The ultrasound-guided transversus abdominis plane (TAP) block has emerged as an effective adjunctive analgesic technique for abdominal surgery. However, its use in older patients with significant ascites has been rarely documented. This report presents the anesthetic management of an older patient with massive ascites undergoing open laparotomy for an ovarian tumor. Preoperatively, 30 mL of 0.2% levobupivacaine was injected into the TAP under ultrasound guidance. The procedure was uneventful, with approximately 9,000 mL of ascitic fluid drained, along with the removal of a 13 × 13 × 7-cm left ovarian mass, an 8 × 5.5 × 4-cm uterus, and a 3.5 × 1 × 0.5-cm right ovary. Throughout the surgery, the patient maintained hemodynamic stability, with no significant fluctuations in blood pressure or heart rate. Postoperatively, the patient reported minimal pain and experienced no adverse effects. These findings highlight the effectiveness of ultrasound-guided TAP block as an auxiliary anesthesia technique, providing enhanced analgesia, promoting hemodynamic stability, and improving overall anesthetic outcomes in older patients with substantial ascites.