Coastal ecosystems are continuously exposed to various forms of environmental pollution, and the Caspian Sea is no exception. This study considered to investigate the abundance, types, and polymer composition of microplastics in sediments and seawater of the Kiashahr coastal area, located in the southern Caspian Sea. A total of 24 sediment samples and 4 seawater samples were collected. Microplastics were extracted using a saturated sodium chloride solution, filtered through 25 µm filter papers, morphologically identified under a binocular microscope, and chemically characterized using Fourier Transform Infrared Spectroscopy (FTIR). The results showed that in sediments, fibers were the most dominant type, mainly originating from fishing activities, followed by films, fragments, and microbeads. The average concentration of microplastics in sediments was estimated at 1083 particles per ton. In seawater, microbeads were the most prevalent, followed by fibers, films, and fragments, with a mean concentration of 430 particles per cubic meter. FTIR analysis showed that the identified polymers mainly consisted of polyester, polyamide, polyketone, and polyethersulfone, suggesting sources related to fishing nets, textile waste, and urban wastewater.