With the rapid economic development and urbanization in China, haze and photochemical smog events have been frequently observed during the last decade. To explore the temporal and spatial pollution characteristics in Ningbo, a medium-sized coastal city located in Yangtze River Delta (YRD) in southeast of China, 24-h PM2.5 (particulate matters with aerodynamic diameters ≤ 2.5 µm) samples were simultaneously collected at five sites (two urban residential sites, two urban coastal sites and one suburban site) in from winter 2012 to autumn 2013. The average PM2.5 concentration was 53.2 ± 30.4 µg m–3. PM2.5 concentration showed a seasonal variation: highest in winter and lowest in summer. The urban residential sites had the highest PM2.5 concentration, followed by the urban coastal sites, and the suburban site the lowest. OM (Organic Matters) and secondary inorganic ions (sulfate, nitrate and ammonium) were the dominant compositions of PM2.5. As a coastal city with industry zones, sources are more complicated than the inland cities by adding ship emissions and interactions between land and sea, marine and atmospheric environments. Positive matrix factorization (PMF) was used to apportion the particle sources. Nine factors were resolved in this study: secondary nitrate, vehicle exhaust, secondary sulfate, coal combustion, industrial emission, ship emission, dust, biomass burning, and aged sea salt, with the average contributions of 26%, 21%, 13%, 12%, 9%, 7%, 5%, 4%, and 3%, respectively. Secondary nitrate and vehicle exhaust were the major sources of PM2.5 pollution in Ningbo. Coal combustion contributed significantly in winter and autumn. Sea salt had a considerable contribution to PM2.5 in summer. This study suggests that to reduce PM2.5 pollution in Ningbo requires not only reduction strategies on the local primary sources, but also inter-regional joint prevention and control of air pollution in the YRD.