A new renieramycin-type marine natural product was isolated from the blue sponge Xestospongia sp. found in the Philippines and Thailand, and its structure was elucidated to be 7-demethylrenieramycin 0 (= 14 alpha-hydroxyrenieramycin S, 1) by comparing its spectral data with those of renieramycins 0 and S. Exposure of a dichloromethane solution of renieramycin 0 to sunlight gave compound 1 along with renieramycin U. Compound 1 showed weak cytotoxicity to several human cancer cell lines.