In order to investigate the impact of three-dimensional effects on large-diameter monopiles used in offshore wind turbines, two-dimensional analytical model for pile-soil interaction was developed for large-diameter monopiles subjected to horizontal loading in various soil types, which considered only the radial and circumferential stress components distributed along the pile shaft. A comparative analysis was then conducted based on this model to investigate the effects of three-dimensional effects on the monopile’s lumped stiffness, the response of the pile shaft (horizontal displacement, rotation angle, shear force, and bending moment), and the distributed spring stiffness. The results indicate that three-dimensional effects significantly influence the force-controlled lumped stiffness, pile deformation, and shear response of monopiles, while their impact on position-controlled lumped stiffness and bending moment distribution is relatively minor. Neglecting three-dimensional effects yields larger pile shaft responses, leading to a more conservative design approach. The three-dimensional effect has little influence on the p-y spring; however, the m-φ spring, which accounts for three-dimensional effects, is the fundamental cause of differences in lumped stiffness and pile shaft response in monopiles. Therefore, the influence of the m-φ spring should be considered when designing large-diameter monopiles.