A star with spectral type S is a late-type giant star (similar to class K5–M) whose spectrum displays bands from zirconium oxide in addition to titanium oxide which is characteristically exhibited by K and M class giant stars. Other s-process elements, for example yttrium oxide and technetium, are also enhanced, clearly indicating neutron capture elements in the period 5 in the periodic system of chemical elements. It may also have features from cyanogen and lithium. Most of these stars are long-period variables.

It has been proposed that most S stars are an intermediate stage in which asymptotic giant stars (AGB stars) transform from ordinary M class giants to carbon stars of class C-N. AGB stars are usually powered by fusion in a shell of hydrogen surrounding an inert core, but during "thermal pulses" helium shell fusion can come to dominate. Other S stars ("extrinsic" S stars) may be evolved, cooler versions of barium stars, where the enhancements of carbon and s-process elements seen in the stars' spectra are relics of mass transfer between members of a binary star system. In these systems, the presently-observed S star has not synthesized the excess carbon and s-process elements itself; that material was produced at an earlier time by its binary companion, which was at the time a carbon star. We are observing these systems long after the mass transfer event, when the companion has evolved to become a white dwarf and is usually not directly visible.