One new isobenzofuranone derivative, 1,4-dimethoxy-3-(3R*-hydroxy-3R*-methyl-1-tetralone)-1(3H)-isobenzofuran (1), was isolated from the broth of marine Streptomyces sp. M268. The structure was elucidated by spectroscopy characteristics as well as comparison with the literature. Compound 1 exhibited cytotoxicities against human cancer cell, HL-60, A549, and BEL-7402.