A new antioxidant, designated pyranonigrin L, was isolated from the culture materials of the hot spring-derived fungus Penicillium adainetzii BF0003 by solvent extraction, ODS column chromatography, and HPLC. Its planar structure was elucidated using various analytical techniques including UV, IR, and NMR spectroscopy and MS. Its absolute configuration was determined by a comparison of its circular dichroism (CD) spectrum with those of structurally related compounds. Pyranonigrin L was found to exhibit anti-oxidative activity with an EC(500 value of 553 μM.