.Leptospirosis, a global neglected zoonotic disease, is an important public health problem in Thailand. Nonspecific symptoms, lack of laboratory confirmation, and underreporting contribute to its neglected disease status. To better understand the distribution of leptospirosis exposure in Thailand, a retrospective leptospirosis seroprevalence study was conducted on repository serum specimens obtained from young Thai men entering the Royal Thai Army during 2007–2008. The overall nationwide leptospirosis IgG seroprevalence among these young Thai men was 28% (95% confidence interval = 26–30%) and the range by province was 10–52% confirming leptospirosis as an endemic disease throughout Thailand. Seroprevalence was highest in individuals with the lowest education from rural areas, and higher seroprevalence was found in the north and south regions contrary to current morbidity reports. Improvement in reporting and surveillance as well as better access to leptospirosis diagnostics will increase leptospirosis awareness and detection and enable more effective public health interventions.