The aim of the present study was twofold: (i) to validate the drone methodology for floating marine macro-litter (FMML) monitoring, by comparing the results obtained through concurrent drone surveys and visual observations from vessels, and (ii) to assess FMML densities along the North Western Mediterranean Sea using the validated drone surveys. The comparison between monitoring techniques was performed based on 18 concurrent drone/vessel transects. Similar densities of FMML were detected through the two methods (16 items km−2 from the drone method vs 19 items km−2 from the vessel-based visual method). The assessment of FMML densities was done using 40 additional drone transects performed over the waters off the Catalan coast. The densities of FMML observed ranged 0–200 items km−2. These results provide a validation of the use of drones to monitor FMML and contribute to increasing the knowledge about the density of FMML in the North Western Mediterranean Sea. Display omitted •We compared results from concurrent visual and drone surveys.•The two methods proved equally effective to detect floating marine macro-litter.•We used the drone method to monitor marine litter along the Catalan coast.•We detected densities up to 200 items km−2 in the area off the city of Barcelona.•These results encourage using the drone method to monitor marine litter.