Erythema nodosum (EN) is a condition in which the skin suffers from oval-shaped and tender erythematous patches, mostly on the anterior region of the leg and sometimes other parts of the body, including the arms and affects subcutaneous adipose tissue. These patches are self-limiting and usually heal on their own within one to two weeks. This is a case report of a patient who is a 14-year-old boy complained of oval-shaped tender erythematous lesions on the legs of both legs and referred and was hospitalized 5 days ago. The patient continued to complain of pain in both thighs when standing and sitting. The patient developed a high fever upon admission. There was no history of the disease while we were taking his history. All laboratory tests were performed and rheumatic diseases and possible malignancies were evaluated and rejected. The patient was treated with the anti- naproxen, an inflammatory drug, and then the patient’s pain and fever decreased. However, the patient suffered from multiple ulcerative lesions in the mouth on the final day of hospitalization. Infectious counseling was performed and he was discharged with a diagnosis of EN in the context of oral herpes. Two days after discharge, the patient suffered from hematochezia. Consequently, he was hospitalized and underwent colonoscopy, which showed multiple mucosal lesions in the esophagus and duodenum, and a sample was sent for pathology. The test result showed Crohn’s disease, and the patient was treated immediately.