•The quantity and composition of suspended microplastics are reported for the first time in China.•Microplastics (0.5–5mm) constituted more than 90% of total plastics by number of items.•Microplastics concentrations were spatially variable in the Yangtze Estuary.•Large rivers are contributed to the high MP abundances in the adjacent marine environment.•The most abundant plastics were fibres, while plastic spherules were rarely found. Levels of microplastics (MPs) in China are completely unknown. This study characterizes suspended MPs quantitatively and qualitatively for the Yangtze Estuary and East China Sea. MPs were extracted via a floatation method. MPs were counted and categorized according to shape and size under a stereomicroscope. The MP densities were 4137.3±2461.5 and 0.167±0.138n/m3, respectively, in the estuarine and the sea samples. Plastic abundances varied significantly in the estuary. Higher densities in three sea trawls confirmed that rivers were the important sources of MP to the marine environment. Plastic particles (>5mm) were observed with a maximum size of 12.46mm, but MPs (0.5–5mm) constituted more than 90% by number of items. The most frequent geometries were fibres, followed by granules and films. Plastic spherules occurred sparsely. Transparent and coloured plastics comprised the majority of the particles. This study provides clues in understanding the fate and potential sources of MPs.