Ten new tanzawaic acids, namely, steckwaic acids A-D (<b>1</b>-<b>4</b>), 11-ketotanzawaic acid D (<b>5</b>), 6,15-dihydroxytanzawaic acid M (<b>6</b>), 15<i>R</i>-methoxytanzawaic acid M (<b>7</b>), 15<i>S</i>-methoxytanzawaic acid M (<b>8</b>), 8-hydroxytanzawaic acid M (<b>9</b>), and 8-hydroxytanzawaic acid B (<b>10</b>), together with four known analogues (<b>11</b>-<b>14</b>), were isolated and identified from the endozoic fungus <i>Penicillium steckii</i> AS-324, which was obtained from a deep-sea <i>Acanthogorgiidae</i> sp. coral that was collected from the Magellan seamount in the Western Pacific Ocean. Structurally, steckwaic acids A-D (<b>1</b>-<b>4</b>) 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 <b>1</b>, <b>5</b>, and <b>9</b> exhibited activities against aquatic pathogenic bacteria <i>Micrococcus luteus</i>, <i>Vibrio anguillarum</i>, and <i>Pseudomonas aeruginosa</i> with MICs of 2, 4, and 4 μg/mL, respectively.