Microplastic pollution is a growing threat to soils, but its effects on plant litter decomposition remains poorly understood. Particularly, it is unclear how the contribution of soil microorganisms and detritivores to litter decomposition is affected by microplastic pollution. To address this knowledge gap, we evaluated the effect of increasing microplastic concentrations on microbial and isopod-driven litter decomposition, separately, in a one-month full-factorial microcosm experiment under controlled conditions. Contrary to expectations, neither decomposition by microorganisms nor isopods were affected significantly by increased microplastic concentrations. Furthermore, isopod body weight remained unaffected by increased microplastic concentrations. This suggests that microplastics pollution has no observable short-term impact on the contribution of neither microbial nor faunal decomposers to plant litter decomposition. This contrasts with few recent studies that reported positive effects on microbial activity, negative effects on detritivore activity, and an overall positive effect of microplastics on litter decomposition. Microplastic type, concentration, exposure time and application mode likely influence microplastic effects on soil processes, and future research should thus focus on longer-term experiments with environmentally relevant microplastic composition and concentrations.