Although preparation conditions are known to affect the morphology and catalytic performance of hydrothermally synthesized MoS₂, the influence of pH remains unclear. Herein, unsupported MoS₂ was prepared from ammonium tetrathiomolybdate (ATTM) by a hydrothermal reaction at various pH values under a reaction pressure of 2 MPa. The physical and chemical properties of the MoS₂ samples were characterized, and the catalytic performance for CO methanation was examined. With increasing pH, the morphology of the MoS₂ particles transformed from aggregates of irregular grain-like particles to flower-like particles through the agglomeration of fine mesoporous nanoflakes. Hydrothermal synthesis at a pH of 9.5 increased the MoS₂ crystallinity by enhancing the stacking of the (0 0 2) lattice plane. The MoS₂ samples prepared at pH 7.0 and 9.5 showed increased CO conversion during methanation, which was associated with a low concentration of Mo⁵⁺ species and the presence of surface sulfate species. Thus, a high pH during catalyst preparation may promote the complete decomposition of ATTM to MoS₂ and the formation of sulfur vacancies, which can facilitate methanation.