The microplastic (MP) contamination of oceans, freshwaters, and soils has become one of the major challenges within the Anthropocene. MP is transported in large quantities through river systems from land to sea and is deposited in river sediments and floodplains. As part of the river system, floodplains and their soils are known for their sink function with respect to sediments, nutrients, and pollutants. However, the questions remain: To what extend does this deposition occur in floodplain soils? Which spatial distribution of MP accumulations, resulting from possible environmental drivers, can be found? The present study analyzes the spatial distribution of large (L-MP, 2000–1000 μm) and medium (M-MP, 1000–500 μm) MP particles in floodplain soils of the Lahn River (Germany). Based on a geospatial sampling concept, the MP contents in floodplain soils are investigated down to a depth of 2 m through a combined method approach, including MP analyses, soil surveys, properties, and sediment dating. The analysis of the plastic particles was carried out by density separation, visual fluorescence identification, and ATR-FTIR analysis. In addition, grain-size analyses and ²¹⁰Pb and ¹³⁷Cs dating were performed to reconstruct the MP deposition conditions. The results prove a more frequent accumulation of MP in upper floodplain soils (0–50 cm) deposited by flood dynamics since the 1960s than in subsoils. The first MP detection to a depth of 2 m and below recent (>1960) sediment accumulation indicates in-situ vertical transfer of mobile MP particles through natural processes (e.g., preferential flow, bioturbation). Furthermore, the role of MP as a potential marker of the Anthropocene is assessed. This study advances our understanding of the deposition and relocation of MP at the aquatic-terrestrial interface.