Objectives: There is a little data about rectal administration of Ketamine as a postoperative analgesic, so we compared the efficacy of rectal ketamine with rectal acetaminophen, which is applied routinely for analgesia after painful surgeries like tonsillectomy. Methods: In this single-blinded comparative trial, we enrolled 70 children undergoing elective tonsillectomy, and divided them randomly in two groups. Patients received rectal ketamine (2 mg / kg) or rectal acetaminophen (20 mg / kg) at the end of surgery. The children's Hospital of Eastern Ontario Pain scale was used to estimate pain in children. Also the vital signs, Wilson sedation scale, and side effects in each group were noted and compared for 24 hours. Results: The ketamine group had a lower pain score at 15 minutes and 60 minutes after surgery in Recovery (6.4 ± 0.8, 7.4 ± 1 vs. 7.1 ± 1.2, 7.8 ± 1.2 in the acetaminophen group, P < 0.05) and one hour and two hours in the ward (7.2 ± 0.7, 7 ± 0.5 vs. 7.9 ± 1.2, 7.5 ± 1.2 in the acetaminophen group, P < 0.05), with no significant differences till 24 hours. Dreams and hallucinations were not reported in the ketamine group. Systolic blood pressure was seen to be higher in the ketamine group (104.4 ± 7.9 vs. 99.8 ± 7.7 in the acetaminophen group) and nystagmus was reported only in the ketamine group (14.2%). Other side effects were equivalent in both the groups. Conclusions: With low complications, rectal ketamine has analgesic effects, especially in the first hours after surgery in comparison with acetaminophen, and it can be an alternative analgesic with easy administration in children after tonsillectomy.