Unlike the known aggregation-caused quenching (ACQ) that the enhancement of π–π interactions in rigid organic molecules usually decreases the luminescent emission, here we show that an intermolecular “head-to-head” π–π interaction in the phenanthrene crystal, forming the so-called “transannular effect”, could result in a higher degree of electron delocalization and thus photoluminescent emission enhancement. Such a transannular effect is molecular configuration and stacking dependent, which is absent in the isomers of phenanthrene but can be realized again in the designed phenanthrene-based cocrystals. The transannular effect becomes more significant upon compression and causes anomalous piezoluminescent enhancement in the crystals. Our findings thus provide new insights into the effects of π–π interactions on luminescence emission and also offer new pathways for designing efficient aggregation-induced emission (AIE) materials to advance their applications.