To investigate the characteristics and sources of PM2.5 in the Yangtze River Delta (YRD) region, a total of 10 sampling sites were selected in the three major cities of Shanghai, Nanjing, and Ningbo and the regional background city of Lin’an, and 380 samples were collected in spring and winter. The spatiotemporal characteristics of the PM2.5 mass concentration and the chemical components were analyzed. Meanwhile, air mass clusters and the positive matrix factorization (PMF) source apportion model were comprehensively used to identify the sources of PM2.5. The mass concentration of PM2.5 in winter (83.51–107.64 µg m–3) was higher than that in spring (54.11–85.72 µg m–3). Sulfate, nitrate, and ammonium (SNA) dominated the PM2.5, accounting for 39.0% in spring and 46.1% in winter in the YRD region. Higher ratio of secondary organic carbon (SOC) to organic carbon (OC) generally occurred in winter due to increased emissions of organic precursors. The mean equivalent ratio of AE/CE was 0.81 ± 0.25 in winter and 0.96 ± 0.33 in spring, which indicated a slight alkalinity of the atmospheric particles in the YRD region. Secondary aerosols (29.44%) and traffic (25.66%) were identified as the main sources of PM2.5 in the YRD during spring, and they contributed 22.71% and 29.71% in winter. In addition, air mass flow from Northern China imported substantial fugitive soils to the YRD region in spring and enhanced the contribution of combustion in winter through long-range transport, while air masses originating from Southwestern China were strongly associated with biomass burning. Our results showed that the PM2.5 concentration and chemical characterization in the YRD region was significantly influenced by air mass transport.