Ten new tanzawaic acids, namely, steckwaic acids A-D (1-4), 11-ketotanzawaic acid D (5), 6,15-dihydroxytanzawaic acid M (6), 15R-methoxytanzawaic acid M (7), 15S-methoxytanzawaic acid M (8), 8-hydroxytanzawaic acid M (9), and 8-hydroxytanzawaic acid B (10), together with four known analogues (11-14), were isolated and identified from the endozoic fungus Penicillium steckii AS-324, which was obtained from a deep-sea Acanthogorgiidae sp. coral that was collected from the Magellan seamount in the Western Pacific Ocean. Structurally, steckwaic acids A-D (1-4) represent the first tanzawaic acid derivatives containing a naphthalene ring. Their structures were elucidated by detailed interpretation of the NMR spectroscopic and mass spectrometric data. X-ray crystallographic analysis and ECD calculations confirmed their structures and absolute configurations. New compounds 1, 5, and 9 exhibited activities against aquatic pathogenic bacteria Micrococcus luteus, Vibrio anguillarum, and Pseudomonas aeruginosa with MICs of 2, 4, and 4 μg/mL, respectively.