Highlights Retroperitoneal cystic tumors are rare. Cystic lymphatic malformation is one of the more common intra-abdominal cystic tumors. We present a case of spontaneous regression of a cystic peripancreatic tumor in a 12-year-old boy. The cystic tumor was initially suspected to be a pancreatic pseudocyst associated with traumatic pancreatic damage. However, the differential diagnosis included the possibility of lymphatic malformation in view of the clinical and image findings. The cystic tumor in this patient was considered to have been caused by lymphatic malformation, which has a very low prevalence. We did not clarify whether the lymphatic malformation was congenital or acquired due to lymphatic obstruction resulting from the trauma or inflammation. In anticipation of spontaneous regression, the patient has been followed without treatment. Elective drainage is available but fortunately the cyst has shown no sign of expansion. Therefore, observation monitoring may be appropriate in some cases.