The effect of the size and volume fraction of Ti₃N₄ particles in Ti-(50.3–51.5) at % Ni single crystals on their martensitic transformation temperatures and temperature hysteresis is studied. Aging at T = 673−823 K leads to a nonmonotonic change in the martensitic transformation temperatures and temperature hysteresis, which is related to a change in the Ni concentration in the matrix, the hardening of the high-temperature phase, a change in the elastic and surface energies generated upon the martensitic transformations, and the internal stresses that appear because of the difference in the lattice parameters of the particles and the matrix. As a result of the high strength of the B2 phase and the high elastic and surface energies that are generated upon the martensitic transformations due to the precipitation of particles of size d < 40 nm at an interparticle distance λ< 50 nm, the martensitic transformation temperatures decrease down to the suppression of the R-B19′ transitions upon cooling to 77 K. A thermodynamic description for the martensitic transformations in heterophase crystals is proposed, and an analogy between the martensitic transformations in heterophase Ti-Ni single crystals with nanoparticles of size d = 20–100 nm and those in single-phase Ti-Ni polycrystals with a grain size of 50–200 nm is found.