Nanocomposites have an important place due to their improved mechanical properties. Nanoclays have attracted interest from researchers due to their features such as a high aspect ratio and low cost. In this study, the effect of nanoclay on the mechanical properties of E-glass/epoxy composites was investigated. The nanoclay was added to the epoxy at the ratios of 1, 3, and 5 wt%. The tensile strength and the modulus of elasticity were found in both directions. In addition, the Poisson’s ratio was obtained. Additionally, the flexural strength, the shear strength, and the interlaminar shear strength were investigated. The damaged surfaces of the tested samples were examined by scanning electron microscopy. According to the results obtained, it was seen that the shear strength, the tensile strength, and the modulus of elasticity in the fiber direction and the tensile strength in the transverse direction decreased for all ratios. While the Poisson’s ratio increased by 16.67% at the ratios of 3 and 5 wt%, the modulus of elasticity in the transverse direction increased by 2.06% at the ratio of 1 wt%. In addition, the flexural strength and the interlaminar shear strength increased 14.63% and 4.78% at the ratio of 1 wt%, respectively. The mechanical properties of the E-glass/epoxy composites are significantly influenced by the nanoclays.