In the search for new food sources that contribute to the optimization of livestock production this paper discusses the possibility of using waste (called banquettes) of a marine plant commonly found on the Mediterranean coasts: Posidonia oceanica. The idea stems from the use of a waste that in summertime generates large costs because it is considered bothersome on the beaches. Thus, tons and tons of residues are collected each year from the beach, being destined for incineration. However, alternative uses for these residues are suggested, such as forage that is particularly relevant for the Mediterranean coast, where the weather does not support abundant grass growth. With this purpose, samples of banquettes of P. oceanica from six different points of a touristic place located in the Murcia Region (S.E. of Spain) were collected in April 2012 on the same day directly from the beach above the water line, washed with distilled water and sun-dried for 48 h. Approximately 500 g of each sample of plant material was chopped and two subsamples of 200 g each were placed in airtight plastic containers and sent to the laboratory for mineral and chemical analysis. This report provides data on the nutritional composition of P. oceanica such as mineral contents (As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) and chemical composition (ash, CP, ether extract (EE), NDF and ADF, respectively) and ADL. Finally, the in sacco rumen disappearance of dry matter (DM), organic matter (OM), CP and NDF were studied. Our results showed that minerals, except for Fe, where within the range of values reported for P. oceanica in other parts of the Mediterranean region. Given the high Fe content further studies assessing the antagonic Fe–Cu interaction and its effect on animal health should be addressed. In relation to chemical composition, it is clear that this seagrass is a poor protein source and has levels of DM degradability at 24 h, similar to those obtained for cereal straw. The information summarized here shows some potential for the use of banquettes of P. oceanica as forage source in ruminant nutrition.