The pile foundation supported on a structure can generate large horizontal loads due to earthquakes, high winds, and wave actions. The behavior of piles when subjected to horizontal load is generally analyzed using the <i>p-y</i> curve and “<i>p</i>-multiplier (<i>P<sub>m</sub></i>),” which is the coefficient of the group pile effect. In this study, the <i>p-y</i> curves and <i>P<sub>m</sub></i> were calculated by analyzing a single pile and group of piles arranged in 3 × 3 installed in cohesive soil using the finite element analysis program, Plaxis 3D. The soil resistance (<i>p</i>) increased as the undrained shear strength of the clay increased and the distance between the pile centers (<i>S/D</i>) increased. In the case of the group pile effect, when <i>P<sub>m</sub></i> was closer to the center of the group pile, the distance between the pile centers was smaller, and <i>P<sub>m</sub></i> was less due to the interference effect of the adjacent individual piles. In conclusion, it was observed that <i>P<sub>m</sub></i> is affected by the location of the individual piles and the distance between the pile centers.