The isolation and structural elucidation of a structurally new desertomycin, designated as desertomycin G (), with strong antibiotic activity against several clinically relevant antibiotic resistant pathogens are described herein. This new natural product was obtained from cultures of the marine actinomycete MSM3, isolated from samples of the intertidal seaweed sp. collected in the Cantabrian Sea (Northeast Atlantic Ocean). Particularly interesting is its strong antibiotic activity against clinical isolates, resistant to antibiotics in clinical use. To the best of our knowledge, this is the first report on a member of the desertomycin family displaying such activity. Additionally, desertomycin G shows strong antibiotic activities against other relevant Gram-positive clinical pathogens such as and . Desertomycin G also displays moderate antibiotic activity against relevant Gram-negative clinical pathogens such as and . In addition, the compound affects viability of tumor cell lines, such as human breast adenocarcinoma (MCF-7) and colon carcinoma (DLD-1), but not normal mammary fibroblasts.