Silicon monosulfide (SiS) is one of the numerous diatomic species that have been detected in the interstellar and circumstellar media. About 1 in 5 of all known interstellar species are diatomics. SiS was first detected in 1975 by Morris et al. in the circumstellar envelope of the carbon star CW Leo, also known as IRC+10216. It has since been detected in other sources, including Sgr B2 by Dickenson and Kuiper and Orion KL by Ziurys.

SiS shares bonding traits with CO, CS, and SiO. Their ground states are all closed-shell singlets with a σ bond and one formal π bond formed between the two unpaired electrons of the triplet ground state atoms, strengthened by additional π bonding due to delocalization of the p² electron pair on O or S into unused p orbital on C or Si.