The identification of new pharmacophores is of paramount biomedical importance, and natural products—particularly those from the marine environment, which harbors unmatched biodiversity and chemical diversity—have recently regained attention. Marine cyanobacteria are prolific producers of bioactive secondary metabolites, many with promising antitumor activities. As part of ongoing investigations to identify new drug leads from Florida cyanobacteria, we report the structure determination and preliminary biological characterization of largazole (1), a metabolite from a Symploca sp. collected from Key Largo, Florida Keys. Symploca species were targeted due to their scarce investigation relative to Lyngbya spp. and prior yield of the clinical trial compound dolastatin 10. Largazole was isolated via bioassay-guided fractionation (solvent partition, silica gel chromatography, reversed-phase HPLC). Structural elucidation using ¹H/¹³C NMR, HRESI/APCIMS (molecular formula C₂₉H₄₂N₄O₅S₃), 2D NMR (COSY, HSQC, HMBC), and MSⁿ revealed a novel chemical scaffold with unusual features: a substituted 4-methylthiazoline linearly fused to a thiazole, a thioester moiety (rare in cyanobacteria), and an unprecedented 3-hydroxy-7-mercaptohept-4-enoic acid unit. Absolute configuration (2S,7R,17S) was assigned by chiral HPLC analysis of degradation products (ozonolysis, oxidative workup, and acid hydrolysis yielding L-valine, (R)-2-methylcysteic acid, and L-malic acid). Largazole potently inhibited the growth of transformed cells (e.g., MDA-MB-231 GI₅₀ 7.7 nM, U2OS GI₅₀ 55 nM) with remarkable selectivity over nontransformed cells (e.g., NMuMG GI₅₀ 122 nM, NIH3T3 GI₅₀ 480 nM)—a selectivity unmatched by tested natural product drugs (paclitaxel, actinomycin D, doxorubicin). The compound’s potent antiproliferative activity and preferential targeting of cancer cells warrant further investigation into its mode of action, cancer chemotherapeutic potential, and biosynthesis.