We investigated the secondary metabolites of the colonial ascidian Diazona chinensis collected from the northwest coast of Siquijor Island, Philippines. We report the structures of diazonamides A (1) and B (2), two unusual halogenated cyclic peptides with potent in vitro cytotoxic activity. The combined organic extract of the lyophilized ascidian was partitioned between hexane and methanol, and the methanol-soluble material was further partitioned between butanol and water. Gel filtration of the butanol-soluble material on Sephadex LH-20 with methanol followed by reverse-phase HPLC (ODS-silica) using 9:1 methanol/water gave pure diazonamides A (1, 54 mg, 0.021% dry weight) and B (2, 132 mg, 0.052%). Their structures were established by a combination of ¹H NMR, ¹³C NMR, and HRFABMS experiments, and single-crystal X-ray diffraction analysis of the p-bromobenzamide derivative of diazonamide B (3). Diazonamides A and B represent an entirely new class of halogenated, highly unsaturated cyclic peptides containing derivatives of at least three common amino acids: a 3,4,5-trisubstituted L-tyrosine (C1-C9), a tryptophan substituted at the 2- and 4-positions of the indole (C18-C27), and an L-valine (C31-C35), forming an extremely rigid framework with essentially no conformational freedom for the polycyclic core. Diazonamide A has potent in vitro activity against HCT-116 human colon carcinoma and B-16 murine melanoma cancer cell lines, with IC₅₀ values less than 15 ng/mL, while diazonamide B is less active.