Clemmensen reduction is a chemical reaction described as a reduction of ketones (or aldehydes) to alkanes using zinc amalgam and hydrochloric acid. This reaction is named after Erik Christian Clemmensen, a Danish chemist.

The Clemmensen reduction is particularly effective at reducing aryl-alkyl ketones, such as those formed in a Friedel-Crafts acylation. With aliphatic or cyclic ketones, zinc metal reduction is much more effective.