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The previously undescribed natural product lumnitzeralactone (<b>1</b>), which represents a derivative of ellagic acid, was isolated from the anti-bacterial extract of the Indonesian mangrove species <i>Lumnitzera racemosa</i> Willd. The structure of lumnitzeralactone (<b>1</b>), a proton-deficient and highly challenging condensed aromatic ring system, was unambiguously elucidated by extensive spectroscopic analyses involving high-resolution mass spectrometry (HRMS), 1D ¹H and ¹³C nuclear magnetic resonance spectroscopy (NMR), and 2D NMR (including 1,1-ADEQUATE and 1,n-ADEQUATE). Determination of the structure was supported by computer-assisted structure elucidation (CASE system applying ACD-SE), density functional theory (DFT) calculations, and a two-step chemical synthesis. Possible biosynthetic pathways involving mangrove-associated fungi have been suggested.