Microplastics (MPs) and nanoplastics (NPs) generated from degraded plastics has been detected in thousands of species, including humans. Particularly, NP particles that enter the bloodstream have been reported to interact with diverse blood cells and may contribute to the development of cardiovascular diseases (CVD). However, related studies remain limited. Therefore, this study aimed to evaluate the effects of polystyrene nanoplastics (PS-NPs) on platelet procoagulant activity and to elucidate the underlying mechanisms. Several types of PS-NPs were tested using rat platelets and found that 50 nm amine-functionalized PS-NPs exhibited the most potent effects and was characterized using dynamic light scattering and electrophoretic light scattering, and platelet uptake of PS-NPs was assessed by flow cytometry. Exposure of platelets to PS-NPs resulted in a loss of mitochondrial membrane potential and the formation of the mitochondrial permeability transition pore (mPTP). This led to the activation of caspase-3, which increased the activity of scramblase resulting the disruption of membrane phospholipid asymmetry. Activated scramblase promoted the externalization of phosphatidylserine (PS), thereby facilitating thrombin generation through the coagulation cascade. In conclusion, PS-NPs significantly increased platelet procoagulant activity through a caspase-3–dependent mechanism. This effect may contribute to excessive thrombus formation and thereby increase the risk of CVDs.