The work is aimed at studying the effect of doping with tin in an amount of $x$ = 1–8% wt. on the mechanical behaviour under uniaxial tension of alloys of ternary system Ti–12%Mo–$x$Sn subjected to complex thermomechanical treatment, consisting of melting, homogenizing annealing, quenching, cold rolling with a compression degree of 90–99% and finishing quenching. The changes in mechanical properties of the ternary alloys depending on the concentration of alloying element are analysed. As experimentally established, an increase in strength indices with a decrease in ductility is noted with an increase in tin content up to 10%. Thus, the increase in yield stress is 20–40%, in ultimate strength is 27–35%, and in ductility is in the range of +10–-30% as compared to the indices for the quenched binary alloy Ti–12%Mo after the similar thermomechanical treatment. Such deformation behaviour and an increase in strength characteristics are due to solid-solution hardening, while the alloy deformation occurs by mechanical twinning and phase transformation, that is confirmed by microscopic and X-ray diffraction studies. High ductility of alloys with tin concentration of 1–6% is due to the simultaneous induction of strain martensite with an orthorhombic lattice and twinning, which is accompanied by a high rate of strain hardening.