Arsine is the inorganic compound with the formula AsH₃. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic. Despite its lethality, it finds some applications in the semiconductor industry and for the synthesis of organoarsenic compounds. The term arsine is commonly used to describe a class of organoarsenic compounds of the formula AsH_3−xR_x, where R = aryl or alkyl. For example, As(C₆H₅)₃, called triphenylarsine, is referred to as "an arsine."

At its standard state, arsine is a colorless, denser-than-air gas that is slightly soluble in water (20% at 20 C) and in many organic solvents as well.^{[citation needed]} Whereas arsine itself is odorless^{[citation needed]}, owing to its oxidation by air it is possible to smell a slight garlic or fish-like scent when the compound is present at above about 0.5 ppm. This compound is generally regarded as stable, since at room temperature it decomposes only slowly. At temperatures of ca. 230 °C decomposition to arsenic and hydrogen is rapid.^{[citation needed]} Several factors, such as humidity, presence of light and certain catalysts (namely aluminium) facilitate the rate of decomposition.