Marine litter represents one of the major anthropogenic pressures affecting marine ecosystems, with evidence of its long-term impacts. Despite the growing availability of monitoring programs, the lack of standardized assessment tools limits the comparison of litter conditions among different areas and the evaluation of ecological impacts. To address this gap, we developed MariLIn (Marine Litter Index), a multimetric index designed to provide an assessment of seafloor marine litter by combining three descriptors: debris typology, abundance, and size. The index was tested along a 7-km coastal sector of the Gulf of Genoa (NW Mediterranean), one of the most anthropized coastlines of the Mediterranean Sea. Marine litter surveys were carried out through 80 standardized visual transects, equally distributed between scuba (5–20 m depth) and Remotely Operated Vehicle (ROV) diving (20–40 m depth). Overall, 146 litter items were recorded and classified according to their origin and characteristics. Plastic debris and fishing-related debris accounted for most of the recorded items, although their spatial distribution differed markedly according to depth and proximity to the Port of Genoa. MariLIn successfully discriminated between contrasting pollution scenarios, highlighting a west-to-east decrease in litter impact in shallow coastal habitats, where land-based sources predominate, while revealing the opposite pattern in deeper areas, where fishing-related debris becomes the dominant component because fishing activities are absent close to the commercial harbour. These results demonstrate the ability of the index to integrate different attributes of marine litter into a single ecological metric while preserving information on the main drivers of contamination. The proposed approach provides a practical, scalable, and transferable tool for seafloor litter assessment, facilitating comparisons among coastal areas and over time, and contributing to ecosystem-based management and the implementation of the Marine Strategy Framework Directive. Future applications across different marine environments will further strengthen its robustness and management potential.