The aim of this study was to assess the microplastic (1-5 mm) and large plastic (>5 mm) contamination levels in the sediments of the Golden Horn Estuary (Halic), Istanbul, Turkey. The seasonal sediment samples were collected from seven stations in the estuary from May 2017 to February 2018. The size class and abundance by the number and weight of the plastics (microplastics and large plastics) within the sediment samples were examined. The total plastic abundance by number was recorded as 566 particles/kg wet sediment and determined to be 115 (20.3%) in spring, 105 (18.6%) in summer, 103 (18.2%) in autumn, and 243 (42.9%) in winter. The most common size was >5 mm plastics (198 particles), followed by microplastics in the range of 1-2 mm (118 particles). The plastic abundance by weight in total was identified as 11.1258 g/kg wet sediment. The results of this preliminary study showed that the Golden Horn Estuary (Halic) was contaminated by plastic debris.