The FS2 radical has been detected in the glow discharge of the SF6 gas by microwave spectroscopy. Both a-type and b-type rotational transitions with the fine and hyperfine structure were observed in the millimeter-wave range of 120-185 GHz. The least-squares analysis for the 440 line frequencies led to the determination of the rotational constants, the centrifugal distortion constants, the spin-rotation interaction constants with the centrifugal distortion corrections, and the hyperfine interaction constants. Especially, the off-diagonal constants (epsilon(ab) + epsilon(ba))/2 and T-ab were determined in good precision because some nearly degenerate energy levels were mixed strongly by the off-diagonal spin-rotation interaction, which separated the energy levels up to 3 GHz and even induced the forbidden transitions to be observed strongly. The spin density on the fluorine atom and the chemical bond moments were derived for FS2, and the possible signs of T-ab and (epsilon(ab) + epsilon(ba))/2 were discussed. The behaviors of the FS2 radical and other related transient species in the SF6-discharge system were compared by using the corresponding rotational transitions observed in the microwave spectra.