Background and aims <p>Stormwater management facilities (e.g., basins, swales, trenches, rain gardens) are increasingly implemented in urban areas to address the challenges posed by growing urbanisation and climate change. These systems retain particles through settling and/or filtration, leading to the formation of distinct sediment layers. This study provides novel insights into the dual origin and transformation dynamics of organic matter (OM) in stormwater sediments, with a focus on operational challenges such as maintenance and the management of dredged materials.</p> Methods <p>Sediments were collected from four urban facilities with contrasting designs and locations. A multidisciplinary analytical framework — combining elemental and granulometric analyses, thermogravimetric analysis, Fourier transform infrared spectroscopy, respirometric and leaching assays — was used to assess OM characteristics and sediment composition.</p> Results <p>While inter-site variability in sediment composition reflects local catchment characteristics, this study reveals that intra-site variability can equal or exceed it. Despite this variability, a consistent presence of OM — derived from both external inputs and in situ biological activity — was observed. The organic fraction exhibited a distinct and recurring bio-physico-chemical signature across all sites, indicating shared compositional and functional traits irrespective of system design, sediment age, or catchment context.</p> Conclusion <p>Stormwater sediments emerge as a distinct hybrid material, situated at the interface between urban soils and organic-rich wastes. A key contribution of this work lies in the identification of a common OM signature across diverse contexts, highlighting the role of OM in governing sediment behaviour, and supporting the development of harmonised monitoring and reuse strategies.</p>