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Highly pure 1T MoS₂ nanosheets were grown at 200 °C by a hydrothermal process. The effects of mild annealing on the structural and physical properties of the MoS₂ were studied by heating the nanosheets in air and vacuum up to 350 °C. It was found that the annealing leads to an increase in resistivity for the nanosheets by 3 orders of magnitude, the appearance of two absorption bands in the visible range, and a hydrophilic to hydrophobic change in the surface wetting properties. Monitoring of the annealing process by Raman spectroscopy indicates that the material property changes are associated with a 1T to 2H MoS₂ phase transition, with activation energies of 517 meV in air and 260 meV in vacuum. This study provides another way to control the electrical, optical, and surface properties of MoS₂ nanosheets for fulfilling the needs of various applications.