Marine-derived fungi have emerged as a source for novel metabolites with a broad range of bioactivities. However, accessing the full potential of fungi under standard laboratory conditions remains challenging. LC-MS-based metabolomics in combination with varied culture conditions is a fast and powerful tool to detect new metabolites. Here, three developmental forms of the marine-derived fungusAspergillus alliaceuswere analyzed and 14 fungal metabolites, including new brominated polyketides (11-14) were isolated.Structure elucidation relied mainly on 1D and 2D NMR techniques and was supported by low- and high-resolution mass spectrometry and DFT-based computations. We sequenced theA. alliaceusgenome, identified the bianthrone-producing biosynthetic gene cluster, and conducted expression analysis on genes involved in sexual development and biosynthesis. TheNCI-60 cell line panel revealed selectivein vitroactivity against triple-negative breast cancer (TNBC) for the halogenated allianthrones and their full anti-proliferative and cytotoxic effects were evaluated in fiveTNBCcell lines.