Herein, we report a new pyrene (PY)-based derivative, 1-(4-(phenylsulfonyl)phenyl)pyrene (1-DPS-PY), whose two crystals (crystal-B and crystal-G) exhibit sky-blue and green excimer fluorescence, respectively. Crystal-B and crystal-G have a similar dimeric π–π PY stacking in the crystals, but with a different single-molecule conformation. The experimental and theoretical investigations demonstrate that the substituent orientation of 4-(phenylsulfonyl)phenyl (DPS) plays a crucial role in modulating the excited-state properties of the PY excimer. The small twist angle (Φ) between DPS and PY facilitates the larger overlap between two PY planes in the excited state, resulting in more red-shifted excimer emission, and vice versa. Moreover, crystal-B shows a photoluminescence (PL) efficiency (η_PL) as high as 77% due to the better discreteness of PY dimer stacking, in contrast to η_PL = 60% of crystal-G with the non-discrete PY dimer stacking. Organic light-emitting diodes (OLEDs) were fabricated with doped and non-doped device structures. The devices showed a gradual emission change from deep blue to sky blue with increasing doping concentration, corresponding to the emission species transformation from a monomer to an excimer. A non-doped device exhibited the best performance with an external quantum efficiency (EQE) of nearly 4.5% and a maximum luminance of 13 733 cd m⁻². These results provide us with a new understanding of the mechanism of the PY excimer, and a new strategy to design the polymorphic PY dimers for high-efficiency excimer fluorescence with tunable emission color.