Background: Costello syndrome as a complex multi organ system anomalies caused by heterozygous de novo missense mutations in the proto oncogene HRAS which its product involved in controlling cell division and growth. Typical features that characterize this phenotype include distinctive coarse facial features, craniofacial anomalies, macrocephaly, low body mass gain, delayed development, short stature, skeletal and cardiac abnormalities with tumor predisposition, especially rhabdomyosarcoma. Aim: We here report one pediatric patients with Costello syndrome and umbilical ligament rhabdomyosarcoma. In this article, we report a case study of an 8-year-old boy born of a non-consanguineous marriage and presenting with mental retardation and delayed development. Physical examination revealed loss of speech, repetitive hand wringing movement, short stature (120 cm), low-set ears, strabismus in eyes, microcephaly [head circumference (HC) = 49 cm] and autistic behaviour. The clinical findings prompted a suspicion of CS. Genetic testing provided a proof with G12S mutation in the exon 2 of HRAS gene. The current work aim was to report the first case of Costello syndrome in the Azeri Turkish population.