We evaluated serum gamma-glutamyltransferase (GGT) and the risk of Parkinson's disease (PD). Using data from the National Health Insurance Service (NHIS) database, we constructed a cohort consisting of individuals aged above 40 years who underwent a health check-up in 2009. After excluding individuals with heavy alcohol consumption, hepatobiliary and pancreatic disorders, and a previous history of PD, each quartile group of baseline serum GGT levels was monitored for the development of PD for 7 years. Adjusted hazard ratios (HRs) for PD were estimated by Cox proportional hazard models adjusting for potential confounding variables. We additionally analyzed the possible interaction between GGT and obesity or metabolic syndrome. Among the 6,098,405 individuals who were included, PD developed in 20,895 individuals during the follow-up (0.34%, 9,512 men and 11,383 women). The top quartile of serum GGT (geometric means, 90.44 IU/L in men and 41.86 IU/L in women) was associated with a lower risk in men (adjusted HR = 0.72 (95% CI: 0.67-0.76)) and a higher risk in women (adjusted HR = 1.30 (95% CI: 1.23-1.37)) using the lower GGT quartiles as a reference. Obesity and metabolic syndrome increased PD risk in both sexes, and there was only a subadditive interaction between serum GGT and obesity in women.