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Secondary inorganic ions are important constituents of PM2.5 that play an important role in global climate change through direct and indirect radiative forcing. To investigate the influence of ship emissions on coastal cities, we launched a cruise ship campaign offshore of the East China Sea (ECS) and two field observation sites in the megacity of Shanghai using 1‐hr time‐resolved water‐soluble ion chromatography from June 3 to June 27, 2017. Three main secondary inorganic soluble ions in the atmosphere of the ECS, that is, non‐sea‐salt sulfate (nss‐SO42−), NO3− and NH4+, were 8.87, 4.94, and 4.65 μg m−3, respectively. While their values at the Pudong (PD) station in urban Shanghai were 6.33, 9.47, and 4.74 μg m−3 and at the Dianshanhu (DSH) station in suburban Shanghai were 7.33, 10.01, and 6.78 μg m−3, respectively. Nss‐SO42− was dominant in the PM2.5 in the atmosphere of the ECS (nss‐SO42−/NO3− = 1.8), and mainly distributed near the Zhoushan Islands of the ECS. This indicated significant contributions from ship emissions. Based on the Community Multiscale Air Quality model, two main sources of nss‐SO42− were identified: emissions and horizontal transport. These comprised over 90% of the nss‐SO42−. The influence of ship emissions reached up to 31.6% of the total nss‐SO42− alongside the cruise ship and near coastal parking ports in the Yangtze River Delta area, suggesting that more attention should be paid to ship emissions. Key Points: Non‐sea‐salt sulfate (Nss‐SO42−) was dominant in PM2.5 on East China Sea, which was distributed around Zhoushan Islands indicating a significant contributor of ship emissionTwo main source processing of nss‐SO42− were categorized using Community Multiscale Air Quality: emission and horizontal transportInfluence of ship emission reached up to 31.6% of total nss‐SO42− alongside ship cruise and near coastal parking ports in Yangtze River Delta area [ABSTRACT FROM AUTHOR]