Diastereoselective photocyclization reactions based on a 6π-system have been demonstrated with terarylene molecules having a P-stereogenic moiety. A chiral benzo[b]phosphole chalcogenide unit was introduced as a central bridging part in a terarylene to form a photochromic dithiazolylbenzo[b]phosphole chalcogenide. The stereoselective photoreaction of terarylenes was controlled by the conformational preference in the photoprecursor open-forms, which was dictated by the intramolecular interactions involving the chiral phosphorus atom and its substituents. Dithiazolylbenzo[b]phosphole oxide showed moderate photocyclization quantum yield and diastereoselectivity, which were significantly improved by replacing the oxygen atom with a sulfur atom showing photon-quantitative reactivity in toluene. The conformational preferences, which play an important role in the photocyclization quantum yield and diastereoselectivity, were thoroughly investigated by means of DFT calculations, X-ray crystallography, NMR measurements and circular dichroism spectra.