Hereditary tyrosinemia type III (OMIM 276710) is a rare inborn error of tyrosine metabolism caused by the deficiency of 4-hydroxyphenylpyruvate dioxygenase (HPD). This metabolic statement is transmitted in an autosomal recessive trait and hitherto about 18 cases presenting with this disease have been reported in the literature. Because of the low prevalence of the disease, the clinical phenotype remains variable and unclear, but the main symptoms are mostly related to the high concentrations of tyrosine and phenolic metabolites, namely mental retardation, ataxia, and seizures. We described the clinical, biochemical, and molecular characteristics of an Iranian female patient with tyrosinemia type III and her 7-year follow-up plan. A novel variant of HPD (609695) mutation (c.759+1 G>A) was identified in a homozygous pattern. Despite not being compliant with the recommended diet, the patient continued to have normal neuropsychiatric development in the follow-up, which questions the efficacy of a low-tyrosine diet.