Mesenteric pseuodycst is a very rare benign childhood tumor, accounting for less than 1 out of 250,000 hospital admissions. We here report a case of giant mesenteric pseudocyst incidentally detected in a 11-year-old boy with acute appendicitis. He complained of persistent abdominal pain for the past 48 hours. He had a history of intermittent pain for several months. Physical examination showed fever and abdominal pain. Ultrasonography showed large peritoneal fluid related to peritonitis probably of appendicular origin. The patient underwent exploratory laparotomy revealing giant abdominal mesenteric cyst and acute appendicitis. Open resection of the cyst and appendectomy were performed. The diagnosis of uncomplicated acute appendicitis associated with mesenteric pseudocyst was made. Preoperative diagnosis of pseudomesenteric cysts is a clinical challenge. Knowledge is essential and suspicion should be maintained in patients with nonspecific symptoms.