Background: Breast surgery, can be associated with significant postoperative pain. Objectives: This study was conducted to compare pectoral nerve block (PECS) and thoracic paravertebral block (TPVB) as supplements to general anesthesia for alleviation of patients' postoperative pain after breast surgery. Patients and Methods: This prospective cohort study included 60 female patients with ASA I-III, and body mass index< 30 kg/m2 undergoing mastectomy. Patients were allocated into two equal groups (PECS and TPVB groups). Both types of nerve block were ultrasound-guided. After 30 minutes, general anesthesia was induced. When the mean arterial pressure (MAP) or heart rate (HR) exceeded 20% of the preoperative value, increments of fentanyl 25 µg were given intravenously at 5 minutes intervals. Paracetamol and ketorolac were used for postoperative analgesia. Postoperative pain intensity was measured using visual analogue scale (VAS). The measured parameters included time to the first call for analgesia, intraoperative and postoperative MAP and HR, intraoperative fentanyl consumption, the amount of analgesics used in the first 24 postoperative hours and patients' satisfaction and complications. Results: The PECS Group showed significantly lower intraoperative fentanyl consumption, longer duration before the first postoperative call for analgesia, lower VAS till 18 hours postoperatively, lower postoperative consumption of both paracetamol and ketorolac, and higher satisfaction score as compared to TPVB group. There were no complications in both groups. Conclusions: Pectoral nerve block was superior to thoracic paravertebral block in terms of intraoperative analgesic consumption, duration of analgesia, postoperative pain score, postoperative analgesic consumption, and patients' satisfaction.