Abstract: In this study, we conducted a comprehensive analysis of the abundance, shape, and composition of microplastics in rivers, nearshore surface seawater, and sediments around the Dapeng peninsula in Shenzhen. Our investigation aimed to explore the combined effects of both land and marine activities on the seasonal distribution of secondary microplastics in the coastal waters. During the summer, microplastic abundance in the cross-sections of rivers surrounding the Dapeng peninsula ranged from 0.30 to 12.95 particles/L, with an average value of approximately 2.53 particles/L. In the seawater surface, the abundance varied from 0.02 to 1.30 particles/L, averaging around 0.27 particles/L. Moving into the autumn, microplastic abundance in river cross-sections was from 0.10 to 0.75 particles/L, with an average of 0.36 particles/L. Meanwhile, the abundance in coastal seawater surfaces ranged from 0.02 to 5.24 particles/L, averaging about 0.70 particles/L. Our findings suggest that the types and distribution patterns of microplastics in coastal waters are primarily influenced by terrestrial human activities during the summer wet season. During the autumn dry season, conversely, the combined effects of both land and marine activities become more prominent. High abundance of microplastics along riverbanks is attributed to sources such as residential areas, storage and transportation, and industrial land. Elevated microplastic levels in the nearshore seawater and sediments are linked to emissions from fishing and ship transportation activities, and with a higher likelihood of accumulation in seabed sediments nearshore.