A prospective cohort study identified Galectin-9 as a biomarker that can forecast the risk of persistent postherpetic neuralgia after acute herpes zoster. Elevated baseline serum Galectin-9 during the acute episode was associated with subsequent risk and severity of the chronic pain condition. For product developers and pain medicine clinicians, biomarker-based early risk stratification could shift intervention timing and help justify preventive strategies for patients most likely to develop prolonged neuropathic pain.