A computationally efficient method was developed to implement the Ewald summation in calculations for Coulomb interactions in three-dimensional (3D) systems with two-dimensional (2D) periodicity. The computational efficiency and accuracy of this method was evaluated for water systems enclosed in various rectangular parallelepiped boxes (cube, quadratic prism, and slab) and with 2D periodicity. Compared with existing computational methods, this method showed a significant reduction in computational time for all systems examined and was sufficiently accurate for calculating the Coulomb interactions in 3D systems with 2D periodicity, independent of the shape of the simulation box.