Herein we report the isolation, structure elucidation, and biological activities of four novel ansa macrolides, divergolides A–D, from an endophyte Streptomyces sp. HKI0576 of the mangrove tree Bruguiera gymnorrhiza. The compounds were purified via column chromatography, size-exclusion chromatography, and preparative HPLC after scaling up fermentation to 200 L due to trace metabolite production. Their structures were elucidated using NMR spectroscopy, X-ray crystallography, and CD spectroscopy, revealing novel architectures: divergolide A has an unusual branched side chain, disrupted polyketide backbone, and unprecedented tricyclic chromophore; divergolide B features an unprecedented benzopyran/chromene nucleus; divergolides C and D have tetracyclic scaffolds. A retro-biosynthetic analysis suggested they derive from an AHBA-primed polyketide backbone disrupted by Baeyer–Villiger oxidation, with diversification arising from flexible polyketide chain reactions including three cyclization types leading to alternative chromophores and regiodivergent heterocyclizations. Biological assays showed varied activities: 1 against Mycobacterium vaccae, 4 against Bacillus subtilis, Staphylococcus aureus, and tumor cell lines (e.g., lung, pancreatic, renal cancer, sarcoma with IC50 1.0–2.0 μM), 3 against Enterococcus faecalis. In conclusion, we isolated and fully characterized four novel ansamycins from the mangrove endophyte through extensive NMR spectroscopic and X-ray analysis. Despite significant structural differences, they share a common biosynthetic precursor. The most remarkable finding is that all four fit into one biosynthetic scheme, with diversification resulting from flexibility in polyketide chain reactions. This degree of "in-built diversification" is unprecedented for complex polyketides and highlights the biosynthetic versatility of nature. To the best of our knowledge, this is the first report of ansamycins from a tree endophyte.