The worldwide production of plastics has been reported to grow from 335 million t in 2016 up to 348 million t in 2017, giving employment to over 1.5 million people in Europe. Plastic materials have changed our way of life because of their versatility, high durability and ability to be moulded in different shapes. For that reason, when discarded in the marine environment, plastics and especially microplastics can become an environmental hazard. This article describes the presence and abundance of microplastics in sandy beaches of a coastal city, Cartagena (southeast Spain), surrounding the Mar Menor coastal lagoon, an important tourist destination with also local activities, mainly fishery and agriculture. Microscopic observations and Fourier-transform infrared spectroscopy analyses displayed a total of 14 polymer families in the microplastic composition, mainly represented by low-density polyethylene (LDPE), high-density polyethylene, polyvinyl ester (PVE), polypropylene (PP), polystyrene, nylon (NYL) and polyester (PES). The extensive amount of polymer types together with an important variety of colours demonstrates the multiple origin of microplastics. LDPE in a film form proved to be a consequence of plastic greenhouses degradation, prone to cracking under environmental stress, because of their transportation through a northwest catchment down to the beach. Similarly, PVE used in naval composite structures as a primary resin proved to be higher in urban than in natural beaches because of the massive use of fishing boats and pleasure crafts. Littering and runoff were the main sources for other microplastic particles, mainly PP, NYL and PES.