Three-dimensional nonlinear liquid motion in a rigid rectangular tank partially filled with liquid, in response to pitching excitation, is investigated. The nonlinear ordinary differential equations governing the liquid surface oscillation are derived by applying Galerkin's method to basic equations. The fundamental modes in two horizontal directions of the tank are considered dominant in the admissible functions that are assumed to be represented by combining the modal functions obtained by the linearization analysis. The nonlinear coupling characteristics of two dominant modes are discussed. Calculations are presented for the time histories of the liquid surface displacement to the harmonic pitching excitation. It is shown that the three-dimensional nonlinear analysis is important for estimating the sloshing responses in a rectangular tank when the aspect ratio of the tank is an integer.