During the course of constructing a natural product library for drug screening consisting of microbial culture extracts originated from marine samples, we evaluated natural product components profiles via UPLC TOF-MS and routine biological tests for cytotoxic and antibiotic activities for all of the culture extract samples. By combination of chemical screening and biological activities, we succeeded in discovering a 20-membered macrolactam antibiotic subsequently designated JBIR-150 (1) from a marine-derived actinomycete identified as Streptomyces sp. that was isolated from an Okinawan marine sediment. The chemical structure of 1 was determined by interpreting NMR spectroscopic and mass spectrometric data. Compound 1 exhibited moderate cytotoxicity against MESO-1 and Jurkat cells.