Marfan syndrome is an autosomal dominant genetic disorder caused by a connective tissue defect. A nine-year-old girl was referred to our pediatric endocrinology clinic for tall stature. Physical examination revealed a lens dislocation with strabismus, high palate, positive wrist and thumb signs, joint hypermobility, and pes planus. Transthoracic echocardiography revealed dilatation of the aortic root. She was diagnosed with Marfan syndrome based on the revised Ghent diagnostic criteria. Molecular investigation identified a heterozygous c.2810G >A variation in the FBN1 gene in the patient, but not in her parents. To our knowledge, this sequence variant has been reported as a polymorphism (rs113602180), but it is the first report identifying it as the genetic cause of Marfan syndrome. We hypothesize that this de novo novel missense FBN1 mutation disrupts fibrillin-1 function and is probably involved in the development of Marfan syndrome in this patient.