We report the detection, synthesis, preparative isolation, structure characterization and identification, and formation of a new group of drinking water disinfection byproducts (DBPs): trihalo-hydroxy-cyclopentene-diones (trihalo-HCDs). With ultra performance liquid chromatography (UPLC)/electrospray ionization-triple quadruple mass spectrometry analyses (full scans, multiple reaction monitoring, and product ion scans) and high-resolution mass spectrometry analyses (full scans), the new group of DBPs was identified with formulae and proposed with structures. However, due to a lack of commercially available standard compounds, structure identification of this new group of DBPs was challenging. 2,4,6-Trihydroxybenzaldehyde was found to be a good precursor for the synthesis of the tribromo species (m/z 345/347/349/351) in the new group of DBPs by reacting with bromine at a 2,4,6-trihydroxybenzaldehyde-to-bromine molar ratio of 1:8. With UPLC/photodiode array analysis (simultaneous 2- and 3-dimensional operations), the new DBP was determined to have a maximum UV absorption at the wavelength of 280 nm. Through isolation with high performance liquid chromatography/UV-triggered collections followed by lyophilization, the pure standard of the new DBP was obtained. Characterized with Fourier transform infrared spectroscopy, the pure standard of the new DBP was finally identified to be tribromo-HCD, and thus the new group of DBPs was identified to be trihalo-HCDs. On the basis of the disclosed structure, formation pathways of tribromo-HCD through reactions of three different precursors and bromine were proposed and partially verified. Moreover, increasing the bromide level in source water shifted the formation of trihalo-HCDs from being more chlorinated to being more brominated; with an increase in the contact time from 1 h to 5 d, the formation of trihalo-HCDs kept increasing in chloramination, whereas they kept decreasing in chlorination; with an increase in the pH from 6.0 to 8.5, the formation of trihalo-HCDs was decreased by ∼80%. Notably, the concentrations of tribromo-HCD in eight Chinese tap water samples were from below the detection limit to 0.53 μg/L.