Using powder x-ray diffraction and first-principles calculations, we provide evidence for a structural transition of PETN-I below 6 GPa to an orthorhombic phase of space group P2₁2₁2. The transition can be rationalized as shear-stress induced and ferroelastic, which involves a slight static displacement of the molecules that breaks the fourfold symmetry of PETN-I. Previously reported changes in the optical spectra reflect a lifting of the twofold degeneracy of modes in tetragonal PETN-I. The observed transition is expected to induce soft shear compliances along specific directions in PETN crystallites that may relate to the observed dependence of detonation pressure on crystal orientation.