Ship emissions are an important factor affecting air quality in coastal areas. The adverse effects of ship emissions on coastal air pollutants are more pronounced in the presence of onshore wind. However, existing studies are based on the summer half of the year and specific stations, so the frequency of onshore winds and their effect on the transport of ship emissions are not fully understood for the whole year or for different seasons. Therefore, in this study, the Yangtze River Delta (YRD) region was selected as the target study area, and the WRF/Chem (The Weather Research and Forecast model coupled with Chemistry) model was used to investigate the impact of ship emissions on PM2.5 in coastal areas under the influence of onshore wind in 2018. There are three main findings. (1) The contribution of ship emissions to PM2.5 under the influence of onshore wind was more significant than either the annual average contribution or the contribution under the influence of non-onshore wind, especially in coastal areas. The contributions of ship emissions to PM2.5 during onshore wind hours reached 9.3 µg m^–3 and 7.4 µg m^–3 in Shanghai and Ningbo, respectively, which were 1.8 and 1.3 times the annual average, respectively. (2) During onshore wind periods, the contribution of ship emissions to PM2.5 increases significantly not only in coastal areas, but also inland. In the central region 200 km from the coast, ship emissions contributed 4.5 µg m^–3 to PM2.5 during onshore wind hours, while the annual average was only 2.5 µg m^–3. (3) Under the influence of onshore wind, the effect of ship emissions on PM2.5 showed obvious seasonal differences. Under the influence of onshore wind, the contribution of ship emissions to PM2.5 was larger in winter than in other seasons. In summer, the contribution of ship emissions to PM2.5 differed the most between onshore and non-onshore wind periods. In spring, the contribution of ship emissions to PM2.5 was greater even though the frequency of onshore wind was the lowest. The contribution of ship emissions to PM2.5 can often be underestimated when only annual or monthly averages are considered. This is detrimental to the accurate prevention and control of ship emissions by relevant authorities during periods of high pollution. The results of this study will help those authorities to understand the transport characteristics of ship emissions in the presence of onshore airflow and provide insights for coastal air pollution control and management.