A new type of disordered substitution alloy of Sb and Te at above 15.1 GPa was discovered by performing in situ high-pressure angle-dispersive X-ray diffraction experiments on antimony telluride (Sb2Te3), a topological insulator and thermoelectric material, at room temperature. In this disordered substitution alloy, Sb2Te3 crystallizes into a monoclinic structure with the space group C2/m, which is different from the corresponding high-pressure phase of the similar isostructural compound Bi2Te3. Above 19.8 GPa, Sb2Te3 adopts a body-centered-cubic structure with the disordered atomic array in the crystal lattice. The in situ high-pressure experiments down to about 13 K show that Sb2Te3 undergoes the same phase-transition sequence with increasing pressure at low temperature, with almost the same phase-transition pressures.