The new radical HS4 containing three sulfur-sulfur bonds has been detected in the gas phase by mass spectrometric experiments. Structures and energies of HS4 in the three oxidation states +/0/- have been investigated by ab initio calculations. HS4 is characterized by the HSSSS open-chain structure, and it is stable toward dissociation into S-2 and HS2 by 23 kcal mol(-1). The first detection of HS4-, the conjugate base of the strong acid H2S4 is also reported.