A new sequential tristhiazole-thiazoline-containing cyclic peptide, marthiapeptide A (1), was isolated from a 60 L scale culture of the deep South China Sea-derived strain Marinactinospora thermotolerans SCSIO 00652. The planar structure and absolute configuration of 1 were elucidated by application of spectroscopic techniques, as well as by single-crystal X-ray diffraction and chiral-phase HPLC analysis of the acid hydrolysates. Marthiapeptide A (1) exhibited antibacterial activity against a panel of Gram-positive bacteria, with MIC values ranging from 2.0 to 8.0 μg/mL, and displayed strong cytotoxic activity against a panel of human cancer cell lines with IC(50) values ranging from 0.38 to 0.52 μM.