Hyperprolinemia type I (HP1) is a rare autosomal recessive disorder caused by biallelic variants in PRODH, encoding the FAD-dependent enzyme proline dehydrogenase. Although persistent hyperprolinemia has been associated with neurodevelopmental and neuropsychiatric manifestations, no established effective treatment is available. Here, we report convergent clinical and patient-derived fibroblast evidence supporting riboflavin responsiveness in HP1 due to a homozygous PRODH c.1397C>T p.(Thr466Met) variant. The patient exhibited markedly elevated plasma proline (530-625 μmol/L; reference 78-273 μmol/L), which decreased after initiation of a vitamin cocktail including riboflavin (291 and 277 μmol/L at 3 and 8 months) and decreased on riboflavin monotherapy (251 μmol/L; 12 mg/day). Urinary P5C became slightly detectable during riboflavin supplementation and was not detectable after riboflavin discontinuation. In patient-derived fibroblasts, intracellular proline was elevated compared with a control and was reduced by riboflavin therapy (40.32 to 14.85 pmol/200 μL PBS; control 21.65 pmol/200 μL PBS). PRODH mRNA showed a modest upward trend under riboflavin in fibroblasts. Although direct enzymatic activity could not be reliably assessed due to technical limitations, these multi-level findings support riboflavin as a potential cofactor-directed therapy in selected PRODH missense variants and provide a rationale for therapeutic trials with biochemical monitoring.