Hydrogen sulfide is the chemical compound with the formula H
2S. It is a colorless gas with the characteristic foul odor of rotten eggs; it is heavier than air, very poisonous, corrosive, flammable, and explosive.

Hydrogen sulfide often results from the prokaryotic breakdown of organic matter in the absence of oxygen gas, such as in swamps and sewers; this process is commonly known as anaerobic digestion. H
2S also occurs in volcanic gases, natural gas, and in some sources of well water. It is also present in natural halite type rock salts, most notably in Himalayan Black Salt, which is mostly harvested from the mineral-rich Salt Range mountains of Pakistan. The human body produces small amounts of H
2S and uses it as a signaling molecule.