By means of first-principles cluster expansion, anisotropic superconductivity in the transition metal dichalcogenide Nb(Se_{x}S_{1-x})_{2} forming a van der Waals (vdW) layered structure is observed theoretically. We show that the Nb(Se_{0.5}S_{0.5})_{2} vdW-layered structure exhibits minimum ground-state energy. The Pnnm structure is more thermodynamically stable when compared to the 2H–NbSe_{2} and 2H–NbS_{2} structures. The characteristics of its phonon dispersions confirm its dynamical stability. According to electronic properties, i.e., electronic band structure, density of states, and Fermi surface indicate metallicity of Nb(Se_{0.5}S_{0.5})_{2}. The corresponding superconductivity is then investigated through the Eliashberg spectral function, which gives rise to a superconducting transition temperature of 14.5 K. This proposes a remarkable improvement of superconductivity in this transition metal dichalcogenide.