Enhancements of the PM_2.5 mass concentration are impacted by many factors such as the source in local and regional areas, the surrounding topography, and the meteorological conditions. In this study, a cluster analysis and a concentration-weighted trajectory (CWT) analysis for both the PM_2.5 and back trajectory data were used to identify the potential source location during the four seasons at an ambient monitoring station in Tokyo during 2012–2014. The results showed that the potential source area affecting the PM_2.5 pollution in Tokyo seasonally varied. For the characteristics of the back trajectories flowing into Tokyo, transportation from the Asian continent and the Pacific coast were identified as the important pathways affecting the PM_2.5 pollution. In the winter, high concentrations of PM_2.5 were observed when the air masses were transported from the Pacific South of Tokyo. These air masses were affected by the combustion source in domestic coastal areas and the accumulated pollutants transported from the regions of Northeast Asia and the Russian Far East. It was suggested that the high concentrations of PM_2.5 in the winter were caused by synoptic weather systems such as circular flow around migratory anticyclones.