Four new N-methoxyindolediketopiperazines (acrozines D—G, 1—4) and six known congeners (5—10) were purified from the organic extract of Acrostalagmus luteoalbus TK-43, which was a marine algal-derived fungus obtained from Codium fragile. Their structures were determined by interpretation of NMR and mass spectroscopic data. The structures of compounds 1, 2, and 4 including their absolute configurations were confirmed by single-crystal X-ray diffraction, while the absolute configuration of compound 3 was elucidated by comparative analysis of ECD and TDDFT-ECD calculations. Compounds 1—4, with a unique methoxy substitution at N-2, were rarely discovered among indolediketopiperazine alkaloids. All these compounds were evaluated for antimicrobial activities against human- and aquatic-pathogenic bacteria and plant-related pathogenic fungi, with compounds 5 and 7 exhibiting potent activity against Edwardsiella icataluri (MIC = 3 and 5 μmol/L, respectively), while compound 9 displayed a broad spectrum of antibacterial activities. The four new compounds were further tested for anti-acetylcholinesterase (AChE) properties, and compound 3 exhibited inhibitory activity with IC50 value of 8.4 μmol/L. © 2021 SIOC, CAS, Shanghai and Wiley-VCH GmbH