Nonlinear and nonplanar lateral vibration of a self-excited vertical cantilevered pipe conveying fluid is studied for the case that the upper end of the pipe is periodically excited in the horizontal direction. The modulation equations, which are coupled with nonlinear terms and govern the amplitudes and phases of nonplanar vibration, are analytically derived. When the excitation frequency is near the nonplanar limit cycle frequency, the nonplanar self-excited vibration is quenched to the excitation, and the amplitude of lateral vibration in the direction perpendicular to the horizontal excitation is decreased. Experiments were conducted and spatial pipe behaviors were observed using two CCD cameras. The theoretically predicted effects of horizontal excitation were confirmed qualitatively.