An acetal is a molecule with two single bonded oxygens attached to the same carbon atom. It is essentially equivalent to a ketal except the acetal has one R group as an H- group. In engineering terms,"acetal" can also refer to polyoxymethylene, an engineering plastic containing acetal groups.

Formation of an acetal occurs when the hydroxyl group of a hemiacetal becomes protonated and is lost as water. The oxonium ion that is produced is then rapidly attacked by a molecule of alcohol. Loss of the proton from the attached alcohol gives the acetal. Acetals are stable compared to hemiacetals but their formation is reversible as with esters. As a reaction to create an acetal proceeds, water must be removed from the reaction mixture or it will hydrolyse the product. The formation of acetals reduces the total number of molecules present and therefore is not favourable with regards to entropy. A way to improve this is to use an orthoester as a source of alcohol. Aldehydes and ketones undergo a process called acetal exchange with orthoesters to give acetals. Water produced along with the acetal product is used up in hydrolysing the orthoester and producing more alcohol to be used in the reaction.

hemiacetal + alcohol + acid (catalyst) <= > acetal + water

Acetals are important in nature, for example in solution the most stable form of glucose is its cyclic hemiacetal and maltose is an acetal made from two glucose units.

Acetals are sometimes used as protecting groups for carbonyl groups in organic synthesis as they are stable with respect to hydrolysis by bases.