Cinnamaldehyde, also known as 3-phenyl-2-propenal or (E)-3-phenylprop-2-enal, is an organic compound with the formula CH3CH=CHCHO. It is a pale yellow liquid with a sweet, floral odor. Cinnamaldehyde is the main component of cinnamon essential oil, and it is also found in other spices such as cassia and allspice.

The molecular structure of cinnamaldehyde consists of a benzene ring attached to a three-carbon chain. The double bond between the second and third carbon atoms forms the aldehyde group, which is responsible for the compound's characteristic odor and flavor.