Background: Hemoglobin Köln is the most widely diagnosed among unstable hemoglobin. Patients with hemoglobin Köln often have moderate hemolytic anemia, reticulocytosis, splenomegaly, and high levels of lactate dehydrogenase and bilirubin in the blood. Hemoglobin Köln happened as a result of the substitution of an amino acid methionine for the usual valine at position 98 of the beta chain. Up to the time, there were no previous reports in Arabian Peninsula, hence, the patient involved in this study is considered to be the first reported case. Case: A 13 years old Saudi male, product of consanguineous marriage, known to have chronic hemolytic anemia and jaundice since birth. Previously labeled as query Gilbert syndrome and treated with folic acid supplements by primary health care. However, due to the several emergency room admissions caused by severe left upper quadrant abdominal pain in association with dark urine, gallstones and hepatosplenomegaly, the patient was referred to pediatric hematology. Subsequent investigations identified that patient's vague and common complaints was a result of hemoglobin Köln disease. Those investigations showed mild macrocytic anemia, polychromasia, and bite cells in blood smear, while in hemoglobin electrophoresis, a small peak in zone (E) was observed. Hemoglobin Köln was confirmed by whole exome sequencing (WES) test that reported the presence of amino acid exchange (Val99Met). Recommendations: Careful examination is necessary for the differential diagnosis of hemoglobinopathy variations, which can be challenging especially in pediatric cases. Unstable hemoglobin should be investigated in cases mimicking a β-thalassemia trait, but associated with abnormally rapid hemolysis and reticulocytosis, or in cases that cannot be explained with common causes.