Background and Purpose: Fatty acid synthase (FAS) is an attractive drug target for the treatment of cancer, obesity and microbial infections. Since FAS has an important role in primary metabolic process, targeting FAS by non-cross-reacting inhibitors will be operationally difficult to implement, though attractive. Experimental Approach: A novel FAS inhibitor, Phormidin, from the marine cyanobacteria, Phormidium ambiguum is reported. Its structure is similar to C75, the FAS inhibitor with a 17 carbon aliphatic chain and an open 6 carbon ring. But, it is comparatively less similar to Celurinin, another known FAS inhibitor lacking the epozxy ring. Key Results: In vitro assay of phormidin cytotoxicity with A549 lung carcinoma cell line showed it to be more active than cerulenin and C75, with an IC50 value 76.034 µg/mL, compared to 86.419 µg/mL for Cerulenin and 99.034 µg/mL for C75. Conclusion and Implications: Enzyme inhibition assay of microbial and mammalian FASs showed promising results, with potential to develop better FAS inhibitor drugs.