Background: As of May 22, 2020, the total number of confirmed COVID-19 cases is over 5 million worldwide and more than 300 thousand people have lost their lives to the virus. South Korea also experienced a sharp increase in late February, but owing to non-pharmaceutical interventions, the number of confirmed cases has been decreasing since March. In this study, we aimed to investigate the transmission dynamics with these effects as well as forecast the spread of COVID-19 in South Korea, using a flexible statistical model.Methods: We analyzed the COVID-19 data obtained at the Korea Centers for Disease Control and Prevention from Feb 18 to April 30. Using a Bayesian susceptible-infectious-hospitalized-removed (SIHR) dynamic model, we estimated the dynamic transmission rate considering the non-pharmaceutical intervention effects and forecast the confirmed cases.Results: The estimated transmission rate without any control effects was 0.4605 with 95% credible interval (0.4468, 0.4745). During the days with effects between February 26 and March 6, the daily transmission rate decreased by about 89.48% of that of the previous day. With consistent control effects, it remained at 0.1549 with 95% credible interval (0.1497, 0.1602). Based on the estimated transmission rate, the forecast number of COVID-19 infections in South Korea showed an overall decreasing pattern.Conclusions: We considered and estimated the dynamic effects of the non-pharmaceutical interventions on COVID-19 using a Bayesian SIHR-based model. This study shows that non-pharmaceutical interventions including active testing, quarantine and isolation, personal preventive measures, and social distancing are crucial to curb the transmission. Our findings contribute to a better understanding of non-pharmaceutical interventions in COVID-19.