Telomeres are the guanine-rich, simple repeat sequences of TTAGGG that constitute the physical termini of eukaryotic chromosomes. Maintenance of telomeres, mediated by the specialized ribonucleoprotein complex telomerase, is significant for immortalization in cancer cells. Given the correlation between telomerase activity and tumors, we screened telomerase inhibitors from microbial metabolites and isolated a potent specific inhibitor designated telomestatin (1) from Streptomyces anulatus 3533-SV4. Telomestatin specifically inhibits telomerase without affecting DNA polymerases (e.g., Taq polymerase) and reverse transcriptases (e.g., HIV-RT). Here, we describe its structure and biological properties: its molecular formula is C26H14N8O7S, and it is the first macrocyclic compound containing a sequential pentaoxazole moiety. Inhibitory assays using semi-purified telomerase from human B lymphoma Namalwa cells showed 1 inhibits telomerase with an IC50 of 0.005 μM, while its IC50 values against HIV-RT and MMLV-RT are 19.4 μM and 13.4 μM, respectively. Compared to synthetic inhibitors TMPyP4 (IC50: 0.63 μM) and BSU1051 (IC50: 80.0 μM), 1 is the strongest and most specific telomerase inhibitor reported. Telomestatin will be a useful tool for studying telomerase's role in cellular senescence and tumorigenic conversion.