A novel metal‐free allylic C−C σ‐bond cleavage of simple olefins to give valuable cinnamyl aldehydes is reported. 1,2‐Aryl or alkyl migration through allylic C−C bond cleavage occurs in this transformation, which is assisted by an alkyl azide reagent. This method enables O‐atom incorporation into simple unfunctionalized olefins to construct cinnamyl aldehydes. The reaction features simple hydrocarbon substrates, metal‐free conditions, and high regio‐ and stereoselectivity. Cut and paste: A novel metal‐free allylic C−C bond cleavage of simple unfunctionalized olefins was developed for the synthesis of valuable cinnamyl aldehyde derivatives with high regio‐ and stereoselectivity. This method not only provides a new approach to cinnamyl aldehydes, but also expands the application of alkyl azides as well as the recombination of olefins in organic synthesis.