This paper is devoted to structure formation and phase transformations in the metastable Ti–15Mo β alloy during high-pressure torsion at room temperature. Transmission electron microscopy and X-ray diffraction analysis show that grain refinement takes place together with a β ↔ ω phase transformation and a nonmonotonic change in the volume fraction of the ω phase when the true strain is increased. The effect of the ω phase on the strength and the elastic properties of the alloy is considered. It is shown that nanostructure formation with a minimum amount of the ω phase is responsible for the marked hardening of the Ti–15Mo alloy while retaining its relatively low elastic modulus.