In this work, molybdenum disulfide (MoS2) nanosheets were employed to enhance the mechanical and thermal properties of chitosan (CS), which was one of the most promising biomaterials. In order to promote a homogeneous dispersion of MoS2 and improved interface interactions between CS and fillers, carboxyl functionalization was subjected to the exfoliated MoS2 nanosheets by chemical conjugation with thioglycolic acid before compositing with CS. Afterward, the CS/MoS2-g-COOH composite films were prepared by a simple solution blending method. Micromorphology characterizations showed that MoS2-g-COOH nanosheets dispersed much better than unmodified MoS2 nanosheets, which was considered to be quite beneficial for the enhanced properties of CS nanocomposites. DMA results indicated that the mechanical properties of CS were improved by incorporating with MoS2-g-COOH nanosheets compared with neat CS and unmodified MoS2-filled CS. The thermal stability of CS/MoS2-g-COOH was also improved. This work demonstrated a promising route to prepare MoS2-based polymer nanocomposites with excellent performances.