Plastic is considered the main component of waste and sometimes accumulates on beaches. The studied stations are located in the coastal areas, approximately 35 km long in the Caspian Sea in Gilan province, Iran. This study aims to quantitatively and qualitatively investigate the presence of microplastics and their combined impact with heavy metals in the environment (water and sediment) as well as in aquatic organisms (specifically Gammarus) within the southwestern area of the Caspian Sea. The results of this research have shown that the highest average concentration of microplastics was found in the water environment samples from the 4th station (13400±658 pieces per liter), while the lowest average concentrations were recorded in the first and fifth stations (5200±370) and (5600±357) pieces per liter, respectively. Microplastics in the form of purple fragments in the size of 0.5 to 1 mm with a magnification of 40 are the most abundant. Heavy metals, notably copper and chromium, have been identified in Gammarus specimens. Additionally, polymers were detected in both water and Gammarus samples, potentially originating from fishing nets and general waste in the area. The presence of microplastics in study stations has increased the concentration of heavy metals in Gammarus and the water environment. The ANOVA test also showed that there is a direct and significant relationship at the 1% level between the concentration of heavy metals and microplastics. These results indicate the mutual and important synergistic effects between microplastics and heavy metals on the marine environment of the Caspian region.