AbstractIsochromans are valuable building blocks for the synthesis of natural products and bioactive compounds. Herein, we report a diastereoselective halo‐cycloacetalization of olefinic aldehydes and alcohols using N‐haloamides as the halogenating agents. The reaction conditions are mild and can be operated in the absence of external catalysts. Olefinic aldehydes bearing different substituents and various alcohols are compatible with the reaction protocol, giving isochroman derivatives efficiently.