Background COVID-19 seroprevalence data has been scarce, especially in less developed countries with a relatively low infection rate. Methods A locally developed rapid IgM/IgG test kit was used for screening hospital staff and patients who required procedural treatment or surgery in 33 hospitals in Thailand from April 8 to June 8, 2020. A total of 587 participants were tested--459 were hospital staff and 128 were pre-procedural patients. (Thai Clinical Trials Registry: TCTR20200426002) Results Overall, 3.4% of the participants (20 of 587) had positive immunoglobulin M (IgM) and none had positive immunoglobulin G. Hospitals located in the central part of Thailand had the highest IgM seroprevalence (7.4%). Preprocedural patients had a higher rate of positive IgM than the hospital staff (7.0% vs. 2.4%). Participants with present upper respiratory tract symptoms had a higher rate of positive IgM than those without (5.9% vs. 2.7%). Three quarter (77%, 452 of 587) of the participants were asymptomatic, of which, twelve had positive IgM (2.7%) which consisted of 7 of 369 healthcare workers (1.9%) and 5 of 83 preprocedural patients (6.0%). Conclusions COVID-19 antibody test could detect a substantial number of potential silent spreaders in Thai community hospitals. Antibody testing should be encouraged for mass screening, especially in asymptomatic individuals.