The effect of the dopant Y on high-pressure-induced polymorph transformation was investigated in Ag₂S nanocrystals. A polymorph transformation from monoclinic P2₁/n phase I (where Ag1 and Ag2 atoms both participated in four-fold-coordinated tetrahedra) to orthorhombic P2₁2₁2₁ phase II (where Ag2 participated in four-fold-coordinated distorted tetrahedra but Ag1 in five-fold-coordinated pyramids) and then to another monoclinic P2₁/n phase III (where Ag1 was still in the five-coordination pyramid-like structure but Ag2 showed a coordination number of five to form trigonal bipyramids) was observed under increasing high pressure at room temperature. The initial monoclinic phase was fully recovered after decompression. Compared with the two transition pressure values of the pure Ag₂S sample, those of the Y-doped sample were found to be lower, revealing the significant influence of even a low Y dopant concentration on the compressibility of Ag₂S. The smaller bulk modulus and larger volume collapse of the Ag₂S:Y sample further suggested it to be more compressible.