Background: COVID-19 is an ongoing pandemic that causes millions of deaths worldwide. Seroprevalence studies play a crucial role in identifying asymptomatic infection and providing the true prevalence of COVID-19 in the community. However, no COVID-19 seroprevalence in Thailand has been reported before. Objective: The study aimed to measure the prevalence of the SARS-CoV-2 antibody among army personnel residing in the Bangkok Metropolitan Area. Methods: All army personnel receiving health checkups from 1 July - 30 September 2020 were invited to participate in the study after providing informed consent. The seroprevalence was conducted using leftover serum without additional venipuncture. The screening conducted using the Rapid test by Wondfo®. When a screening test was positive, a confirmation test would be performed using ELISA by EuroImmun®. In case of a positive ELISA confirmation test result, the COVID-19 investigation team would be activated and deployed. Results: In all, 6,651 army personnel participated in this study. The age of participants ranged from 20-60 years with mean age of 40.5+12.02. Most participants were male (85.5%). The rapid screening test using Wondfo® was positive in 41 cases (0.61%). The confirmation test using ELISA yielded a positive result in 1 subject (0.015%). That person was a known case of COVID-19 infection, who received a full course of treatment and was confirmed to have negative RT-PCR before being discharged from the hospital a few weeks earlier. Conclusion: This was the first large scale seroprevalence surveillance of COVID-19 in Thailand. Our study revealed no new detectable case of asymptomatic COVID-19 infection in the Bangkok Metropolitan Area.