Cu₂Se exhibits a high figure-of-merit (ZT) >1 due to the scattering of phonons by mobile Cu⁺ ions, but migration of the Cu⁺ ions leads to chemical instability at high temperature. It is demonstrated that the incorporation of W in the Cu₂Se matrix results in peak ZT of 2.1 (ZT_avg = 0.93) at 878 K. After the first thermal cycle, the peak ZT of the composite decreases to stable value of ≈1.5 (Z_Tavg = 0.97). The enhanced stability of the nanocomposite along with high ZT_avg is attributed to the covering of Cu₂Se grain boundaries with W that inhibits Cu⁺ migration, reduces Se loss, and improves charge carrier mobility.