AbstractSelf‐trapped exciton (STE) induced broad‐band emission (BE) has sparked considerable interest due to its potential applications in white‐light emitters and optoelectronics. This phenomenon is widely observed in organic–inorganic hybrid perovskites with soft lattice structures, and its physical origin is still under debate. Herein, strong sub‐bandgap STE emission with a large Stokes shift and a photoluminescence quantum yield of up to 9.2% in van der Waals (vdW) layered In_4/3P₂S₆ is reported. Combining comprehensive optical characterizations and theoretical calculations, this concludes that defect‐assisted extrinsic STE is responsible for the BE. The excitonic state can be further localized by hydrostatic pressure, resulting in a threefold PL intensity enhancement. In addition, angle‐resolved polarized Raman demonstrates the anisotropic lattice dynamics in IPS, which may underpin the highly linear anisotropy of the STE emission. This work clarifies the defect, STE, and anisotropy coupling effect in vdW crystal, and provides innovative avenues to modulate the STE luminescence.