In organic chemistry, an acid chloride (or acyl chloride) is very reactive derivative of a carboxylic acid. With the formula R-COCl they are often prepared by reacting a carboxylic acid with thionyl chloride (R-COOH + SOCl₂ -> R-COCl + SO₂ + HCl). The SO₂ and HCl generated are both gases which can leave the reaction vessel, driving the reaction forward, they are also both toxic gases.

Acid chlorides are very reactive, the presence of the chlorine on the carbonyl carbon makes it a very good electrophile, since both the oxygen and chlorine are strongly electron withdrawing, giving the carbon a large partial positive charge. This means even a weak nucleophile can attack the carbon. Since a chloride ion is a good leaving group, this is a very useful compound synthetically, it can be used to prepare any carboxylic acid derivative: an acid anhydride, an ester or an amide by reacting acid chlorides with: a salt of a carboxylic acid, an alcohol or an amine respectively. Acid chlorides will also react with water or a Gilman reagent (lithium diorganocopper compound).