Background: Melatonin became a part of multimodal analgesia in several recent studies because of its analgesic, anxiolytic and anti-inflammatory properties. Incidence of anxiety and pain in patients who underwent hysterectomy are not low. Moreover, preoperative anxiety was related to postoperative pain. The hypothesis of this study was whether melatonin could improve pain and other postoperative conditions after hysterectomy.Methods: A randomized, double-blinded, placebo-controlled trial recruitment of 54 patients, aged between 18 and 65 years old, planned to undergo hysterectomy, with or without oophorectomy under spinal anesthesia. The patients were allocated to receive 4 mg prolonged-release melatonin at night before surgery and in the morning before surgery or 2 doses of placebo at the same time point. Morphine consumption within 24 hours with patient-controlled analgesia machine and visual analog scale (VAS) pain score were recorded. In addition, quality of sleep, Thai standard anxiety level score, fatigue, general well-being and satisfaction score were measured by a blinded assessor and analyzed.Results: Mean of cumulative dose of morphine consumption in melatonin and placebo group were 33.04 ±10.42 and 42.63 ±8.21 mg, respectively. (p < 0.001) Mean of postoperative VAS pain scale was lower in the melatonin group at recovery room arrival (T0) (23.41 vs 8.07, p = 0.01). However, there was not a significant difference between postoperative groups at 1(T1), 6 (T6) and 24 h (T24). Fatigue and general well-being score in the melatonin group were better than the placebo group.Conclusion: Prolonged-release formulation of melatonin decreased pain intensity in post anesthetic care room and lowered doses of postoperative morphine within 24 hours after surgery. Postoperative fatigue, general well-being and satisfaction scores were better in the treatment group. However, there was no anxiety and sleep quality improvement. Melatonin may be an additional choice of multimodal analgesia for hysterectomy. Clinical trial registration: TCTR20140516001, Registered 16 May, 2014, http://www.clinicaltrials.in.th/index.php?tp=regtrials&menu=trialsearch&smenu=fulltext&task=search&task2=view1&id=1076, Efficacy of melatonin on postoperative outcomes after hysterectomy: a randomised, doubl-blinded, placebo-controlled trial