Prohormone convertase 1/3, encoded by the proprotein convertase subtilisin/kexin type 1 gene, is essential for processing prohormones; therefore, its deficiency is characterized by a deficiency of variable levels in all hormone systems. Although a case of postprandial hypoglycemia has been previously reported in the literature, prohormone convertase insufficiency with type 2 diabetes mellitus has not yet been reported. Our case, a 14-year-old girl, was referred due to excess weight gain. She was diagnosed as having type 2 diabetes mellitus based on laboratory test results. Prohormone convertase deficiency was considered due to the history of resistant diarrhea during the infancy period and her rapid weight gain. Proinsulin level was measured as >700 pmol/L(3.60-22) during diagnosis. In genetic analysis, a c.685G> T(p.V229F) homozygous mutation in the PCSK1 gene was detected and this has not been reported in relation to this disorder. In conclusion, patients with recurrent resistant diarrhea during infancy followed by rapid weight gain need to be evaluated with the diagnosis of prohormone convertase deficiency.