Microplastics (MPs) are pervasive contaminants in the aquatic environment, posing significant risks to global drinking water safety. In the sand-bed river, the complex interactions between MPs and sediment regulate downstream MP mobility and can potentially turn riverbeds into long-term sinks or sources of MPs. Here, we conducted controlled flume experiments to quantify the infiltration of MPs into sandy beds and their influence on bedform migration. Through controlled flume experiments, we quantified MP infiltration into sandy beds and its impact on bedform migration. Our results reveal that MPs larger than the sediments accelerate bedform migration even at low concentrations (≥1.2%). The rate of bedform migration changes with increasing MPs-to-sediment volume ratio, and the relationship between MPs-to-sediment and bedform migration rate does not seem to be monotonous. The lower density of MPs compared to sand alters sediment physical properties, thereby modifying flow-driven transport processes. Topographic analysis under identical flow conditions showed no significant change in bedform height after MP introduction, whereas wavelength exhibited a slight increase with higher MP-to-sediment ratios. These findings provide new insights into the MPs transport dynamics in a sand-bed river and inform predictive models of bedform migration in MP-contaminated sediments.