The structure of a new chlorinated disinfection byproduct (DBP) in drinking water is studied. The new DBP was prepared through chlorinating its best precursor, and it was extracted and isolated from the chlorination products. Its structure is identified as 2,2,4-trichloro-5-methoxycyclopent-4-ene-1,3-dione (TCMCD) through FTIR, MS, 1H NMR, and 13C NMR spectroscopic analysis and single-crystal X-ray diffraction method. Results of the Ames test with tester strain TA100 indicated that the newly identified DBP TCMCD is a mutagen.