Hydrogen sulfide (British English: hydrogen sulphide) is the chemical compound with the formula H₂S. It is a colorless, very poisonous, flammable gas with the characteristic foul odor of rotten eggs. It often results from the bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers; this process is commonly known as anaerobic digestion. It also occurs in volcanic gases, natural gas, and some well waters. The human body produces small amounts of H₂S and uses it as a signaling molecule.

Hydrogen sulfide is slightly heavier than air; a mixture of H₂S and air is explosive. Hydrogen sulfide and oxygen burn with a blue flame to form sulfur dioxide (SO₂) and water. In general, hydrogen sulfide acts as a reducing agent.

At high temperature or in the presence of catalysts, sulfur dioxide can be made to react with hydrogen sulfide to form elemental sulfur and water. This is exploited in the Claus process, the main way to convert hydrogen sulfide into elemental sulfur.