Rett syndrome is an X-linked dominant disorder frequently caused by the mutations in the methyl-CpG-binding protein 2 gene (MECP2). Its prevalence in the population is 1/15,000-20,000. Patients with Rett syndrome present apparently normal psychomotor developments during the first 6-18 months of life. Subsequently, they show a short period of developmental stagnation followed by a rapid regression in language and motor development. Precocious puberty is characterized by premature breast and pubic hair development, and advanced bone age development at 8 years of age. We present a case of Rett syndrome and precocious puberty in a 6-year-old girl. At the age of 6, the first signs of precocious puberty appeared (Tanner stage 3). Laboratory measurements were detected as follows: luteinizing hormone (LH), 0.2 mIU/mL; follicle-stimulating hormone (FSH), 1.1 mIU/mL; estradiol, 36 pg/mL; bone age, 9 years. The response to luteinizing hormone releasing hormone (gonadotropin-releasing hormone stimulation test) was characteristic for true precocious puberty (LH, 32 mIU/mL; FSH, 26 mIU/mL). This is the first reported case of precocious puberty related to Rett syndrome.