This study analyzed the trends in change and influencing factors based on particulate matter data from subway stations in 2019 and 2021 (before and during COVID-19). The measurements were based on subway stations on the Seoul Metro from lines 1 to 9. To confirm the changes in particulate matter concentrations before and after COVID-19, the trends of PM10 and PM2.5 concentrations in subway stations in 2019 and 2021 were analyzed. A correlation analysis was subsequently performed for a comparison of the factors influencing changes in the particulate matter concentrations. Overall, both PM10 and PM2.5 concentrations decreased during the outbreak compared to before the COVID-19 outbreak, with rates of 20.7% and 22.8%, respectively. In conclusion, the PM10 and PM2.5 concentrations in the Seoul Metro subway stations during COVID-19 were reduced. The reduction in PM10 and PM2.5, during the COVID-19 period, was the result of government policies such as telecommuting recommendations, subway congestion forecasting systems, and increased ventilation.