Growing concerns about plastic pollution and climate change worldwide are driving research into the effects of microplastics on carbon cycling. Although the impacts of microplastics on soil organic carbon (SOC) levels and carbon emissions have been demonstrated, their role in SOC sequestration remains understudied. Here we discuss how interactions between microplastics and organic matter may affect SOC sequestration through biological, geochemical and physical processes. Microplastics and co-occurring contaminants alter SOC content and quality by leaching dissolved organic matter (DOM) and affecting native SOC mineralization. These changes may influence the efficacy of the microbial carbon pump and mineral-associated organic carbon formation. Furthermore, microplastic surfaces adsorb DOM for long-term preservation and form hotspots for mineral-organic matter interactions, thereby altering carbon storage within soil aggregates. Consequently, a comprehensive understanding of the role of microplastics in soil carbon dynamics, from local to global scales, is essential to address both microplastic pollution and carbon neutrality.