We present a two-dimensional tungsten disulfide (WS2)-based material as a reinforcement additive to produce thermally stable, mechanically strong, and light-weight epoxy (EP) composites. An aqueous melamine solution was used to exfoliate the bulk WS2 into nanosheets while simultaneously functionalizing them to obtain melamine-functionalized WS2 (N-WS2) nanosheets. These non-covalently functionalized N-WS2 nanosheets were used as toughening agents for epoxy composites. The contents of the N-WS2 nanosheets were maintained at below 1 wt%. The effect of functionalization on the interfacial properties of EP composites was investigated in terms of the interfacial interactions between the EP and the nanosheets. The results demonstrated that the surface free energy of EP composites increased significantly after the incorporation of N-WS2 nanosheets into the EP matrix and further increased with the increased loading of N-WS nanosheets up to a maximal value of 54.38 mJ-m2 for the composite containing 0.18 wt% N-WS2 nanosheets, suggesting a stronger interfacial interaction between the EP and N-WS2 nanosheets. A gradual decrease in the surface free energy of the EP composites was observed at higher loadings, which might be attributed to the aggregation of N-WS2 nanosheets within the matrix or excessive bonding of the sheets with one another rather than with the EP matrix due to their larger Van der Waals forces and their large surface area. The mechanical properties of the EP composites were analyzed and demonstrated 55%, 101%, 44%, and 15% improvements in the fracture toughness, fracture energy, flexural strength, and flexural modulus, respectively, for the composite containing 0.18 wt% N-WS2 nanosheets. Moreover, the composites displayed increased glass transition temperatures and better thermal stabilities.