With rapid economic development in China, the concentration of particulate matters emitted into the atmosphere has become increasingly higher. As an important city in the Yangtze River delta, Nanjing City in China has a high PM2.5 concentration level, due partly to the long-range transport of PM2.5 from elsewhere. It is important to study the transport pathways and PM2.5 source areas as the results can serve as scientific evidence for prevention of regional air pollution. In this study, we analyzed the 2013–2016 PM2.5 concentration data at Xianlin in Nanjing City, China. After being clustered to determine the transport pathways, backward trajectory data underwent potential source contribution function (PSCF) and trajectory sector analysis so as to identify potential PM2.5 source areas. A new index called pollution source strength was proposed to indicate the level of pollution in an administrative region. Our results indicate that the concentration of PM2.5 at Xianlin in the city and externally sourced PM2.5 are high in winter, but much lower in summer. The external source areas also show some spatiotemporal regularities. External PM2.5 source areas lie chiefly to the south and southwest of Xianlin, Nanjing City in spring, but mostly to the southeast in summer, to the southeast and northwest in autumn, and to the northwest and northeast with a broader distribution in winter.