Many studies have been conducted to examine the factor of safety of a slope reinforced by a row of piles and the forces acting on these piles. This paper presents an analytical approach to calculate the forces acting on piles and the corresponding factor of safety of slopes stabilized by a row of piles. The proposed approach is based on force equilibrium within the upslope wedge above the pile location and would require knowledge of the upper wedge weight before application. The validity of the proposed analytical approach was verified by comparing the results with those obtained using available approaches in the literature as well as from physical experiments and numerical analyses using available software. Additionally, the effect of arching phenomenon was examined and the optimum pile spacing for use in reinforcing slopes was investigated. The optimum pile spacing depends on the properties of the soil comprising the slope, but the most cost-effective pile spacing was found to be between 4-5 times of the pile diameter, corresponding to the largest spacing that can generate arching between piles. Thus, the proposed analytical approach can be used in practical applications to determine the stability of slopes reinforced by a row of piles.