(also cinnamic aldehyde, β-phenylacrolein), C₆H₅CH=CHCHO, a fatty-aromatic unsaturated aldehyde; a colorless liquid with the characteristic odor of cinnamon. It has a boiling point of 252°C and a density of 1.110 g/cm³ (at 20°C). It is poorly soluble in water and very soluble in alcohol and ether.

Cinnamaldehyde is a component of many essential oils (cinnamon oil and others). In industry it is prepared by the condensation of benzaldehyde with acetaldehyde in the presence of bases. Cinnamaldehyde serves as an aromatic substance in the manufacture of perfumes and used in the preparation of cinnamyl alcohol; the latter is also used as an aromatic substance.