DNAJC12 deficiency (OMIM# 617384) is a new cause of hyperphenylalaninemia (HPA). The deficiency of the co-chaperone protein DNAJC12 was identified in 2017. To date, only 43 patients have been reported. Here, we report four patients from a single family with DNAJC12 deficiency while being followed up with a diagnosis of HPA. Two of the patients, who were cousins, were diagnosed with HPA by newborn screening. And the other two patients were siblings of these patients. Neurological examinations were normal except for one patient with mild learning disability. A c.158-2A>T p.(?) biallelic pathogenic variant was detected in intron 2 of the gene. In the 24 h tetrahydrobiopterin (BH4) challenge test, there was a significant decrease in phenylalanine levels, especially at the 16th hour. Three patients had decreased homovalinic acid (HVA) and 5-hydroxyindoleacetic acid (5HIAA) in cerebrospinal fluid (CSF), while only one had decreased 5HIAA. In treatment, sapropterin, levodopa/carbidopa and 5-OH tryptophan were started. We propose that it will be beneficial to evaluate the patients who have unexplained hyperphenylalaninemia for DNAJC12 deficiency. Patients with early diagnosis of neurotransmitter deficiency may be given a chance to be treated before clinical symptoms begin.