Compared to technical infrastructure, nature-based solutions (NBS) strive to work with nature and to move beyond business-as-usual practices. Despite decades of research from various academia fields and a commencing mainstreaming of the term, a lack of cohesiveness and pertinent methods regarding the subject matter hinders further implementation. Using a functional landscape approach, this paper aims to identify the spatial extent of existing and potential NBS locations and applies it across a case study in Germany. Inspired by hydrological models, which work with delineated hydrological response units, this research instead defines hydromorphological landscape units (HLU) based on biophysical spatial criteria to identify the potential areas that could function as NBS. This approach was tested for floodplain-based NBS. The identified HLU were then compared with historical floodplain and land-use data to differentiate between active or potential NBS. The spatial delineation identified 3.6 million hectares of already active floodplains areas, for which we recommend continued or modified protection measures, and 0.4 million ha where the hydromorphological conditions are apt to support floodplains, yet are cut-off from the flooding regime and require rehabilitation measures. The identification of NBS through explicitly defined HLU serves as a spatial approach to support NBS implementation. Taken together, our research can provide an essential contribution to systemize the emerging scholarship on NBS in river landscapes and to help in selecting and planning appropriate NBS in practice.