A series of rare-earth metal disulfides LnS2 and diselenides LnSe ₂ (Ln = Sm, Gd, Tb, Dy, Ho, Er and Tm) has been prepared by high-pressure high-temperature synthesis at 8(1) GPa. The dichalcogenides LnX₂ (X= S, Se) are found to crystallize in the α-LaS ₂ type with the monoclinic space group P12₁/a1 (no. 14). The crystal structure is a twofold superstructure of the ZrSSi aristo-type and consists of puckered cationic [LnX]⁺ double slabs which are sandwiched by planar chalcogen sheets containing X₂^2- dumbbells.