The occurrence and fate of microplastics in forest ecosystems is a recognized knowledge gap. In this paper, we used an aligned extraction method to quantify microplastics (>20 µm) in organic and mineral forest soil horizons and throughfall deposition. Calculation of forest soil microplastic stocks and throughfall fluxes allowed an estimation of throughfall contribution to microplastic accumulation in forest soils back to 1950. We identified a short-term microplastic enrichment in decomposed litter horizons followed by an accumulation in lower mineral soil caused by litter turnover processes. Similar microplastic features in soil and throughfall deposition indicate that microplastics entering forest soils primarily originate from atmospheric deposition and litter fall, while other sources have a minor impact. We conclude that forests are good indicators for atmospheric microplastic pollution and that high microplastic concentrations in forest soils indicate a high diffuse input of microplastics into these ecosystems.