Background: Quadratus lumborum block (QLB) was introduced to provide postoperative analgesia. Objective: This randomized controlled, double-blind study aimed to evaluate bilateral posterior QLB for analgesia after cesarean section (CS) using spinal anesthesia without intrathecal morphine. Methods: One hundred ASA-II women with singleton, full-term pregnancy undergoing CS using spinal anesthesia were randomized into 2 groups (Quadratus Lumborum and Control; 50 subjects each). After completing CS, bilateral posterior QLB (20 mL bupivacaine 0.25% on each side) was performed in Quadratus Lumborum group, and bilateral sham procedure was performed in Control group. Results: The median (range) of pain score at rest at 6 h was 2 (1-6) in Quadratus Lumborum group and 5 (2-8) in Control group; median difference (95% CI) was -3 (-3.000007 to -2.99995); P <0.00001. Pain score at rest at 12 h and pain scores on movement at 6 and 12 h were lower in Quadratus Lumborum group. No significant differences in pain scores at rest and pain scores on movement at 24 h were detected between the groups. The number of subjects requesting morphine was smaller, time to the first request of morphine was longer, and total morphine dose was smaller in Quadratus Lumborum group. Patient satisfaction was greater in Quadratus Lumborum group. No significant differences in the incidence of pruritus and nausea/vomiting were detected between the groups. Conclusions: QLB provides adequate postoperative analgesia after CS using spinal anesthesia without intrathecal morphine.