As a new acidic selector (resolving agent), we synthesized an enantiopure O‐alkyl phenylphosphonothioicacid with a seven‐membered ring ((R)‐5), which was designed on the basis of the results for the enantioseparation of 1‐arylethylamine derivatives with acyclic O‐ethyl phenylphosphonothioicacid (I). The phosphonothioic acid (R)‐5 showed unique chirality‐recognition ability in the enantioseparation