| Summary: | Three new thiodiketopiperazines, geospallins A⁻C (<b>1</b>⁻<b>3</b>), together with nine known analogues (<b>4</b>⁻<b>12</b>), were isolated from the culture of the deep-sea sediment-derived fungus <i>Geosmithia pallida</i> FS140. Among them, geospallins A and B (<b>1</b> and <b>2</b>) represent rare examples of thiodiketopiperazines featuring an S-methyl group at C-10 and a tertiary hydroxyl group at C-11. Their structures were determined by high-resolution electrospray mass spectrometry (HRESIMS), spectroscopic analyses, and electronic circular dichroism (ECD) calculations. Their angiotensin-converting enzyme (ACE) inhibitory activity was reported, and geospallins A⁻C (<b>1</b>⁻<b>3</b>) showed inhibitory activity with IC<sub>50</sub> values of 29⁻35 µM.
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