Bartter syndrome (BS), a rare autosomal recessive disorder affecting renal tubular potassium handling, is characterized by hypokalemia, metabolic alkalosis, and renal salt wasting. In this report, we describe an adult patient with longstanding clinical symptoms of fatigue, polyuria, polydipsia, mental retardation, and physical dysplasia along with hypokalemia and metabolic alkalosis as laboratory findings. With these clinical symptoms, a patient can be diagnosed with BS type III. Renal biopsy and genetic testing were performed for further confirmation of the diagnosis, revealing renin granular deposits in the juxtaglomerular apparatus (JA) with JA hyperplasia. DNA sequencing detected a heterozygous synonymous mutation, c.1140G>A, in exon 12 of the CLCNKB gene, which could be traced back to a heterozygous synonymous mutation in the patient's mother, who does not have BS.