A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.

The dianion S²⁻ exists only in strongly alkaline aqueous solutions. Such solutions can form by dissolution of H₂S or alkali metal salts such as Li₂S, Na₂S, and K₂S in the presence of extra hydroxide. The ion S²⁻ is exceptionally basic with a pK_a > 14. It does not exist in appreciable concentrations even in highly alkaline water, being undetectable at pH < ~15 (8 M NaOH).

Instead, sulfide combines with protons to form HS⁻, which is variously called hydrogen sulfide ion, hydrosulfide ion, sulfhydryl ion, or bisulfide ion. At still lower pH (<7), HS⁻ converts to H₂S, hydrogen sulfide.

Sulfides are moderately strong reducing agents. They react with oxygen in the air in elevated temperatures to form higher-valence sulfur salts, such as sulfates and sulfur dioxide.

Aqueous solutions of transition metals cations react with sulfide sources (H₂S, NaHS, Na₂S) to precipitate solid sulfides. Such inorganic sulfides typically have very low solubility in water, and many are related to minerals with the same composition (see below). One famous example is the bright yellow species CdS or "cadmium yellow". The black tarnish formed on sterling silver is Ag₂S. Such species are sometimes referred to as salts. In fact, the bonding in transition metal sulfides is highly covalent, which gives rise to their semiconductor properties, which in turn is related to the deep colors. Several have practical applications as pigments, in solar cells, and as catalysts.