The combined effects of temperature and moisture variations, known as hygrothermal effects on the bending of a rectangular multilayer composite plate are investigated based on first-order shear deformation theory. The plate is subjected to transverse mechanical loading, temperature variation, and moisture diffusion. The time-dependent moisture distribution through the plate thickness is obtained using an analytical solution of Fick’s moisture diffusion equation. Owing to the slow moisture diffusion process in polymer-matrix composites, the plate bending is investigated quasi-statically based on the moisture distribution at a specified time. Using the semi-analytical Ritz method and the principle of minimum potential energy, the plate deflection is determined for different boundary conditions. The governing equations and solution method are validated by comparison with published results, followed by analyses of new cases. The effects of various parameters, including the length-to-width and length-to-thickness ratios, asymmetric stacking sequences, boundary conditions, and asymmetric moisture boundary conditions, on the plate deflection are investigated. The results show that increasing the length-to-width ratio increases the contribution of hygrothermal loading relative to mechanical loading to the plate deflection, whereas increasing the plate thickness reduces this contribution. In addition, in most cases, the moisture-induced deflection at one-tenth of the saturation time is greater than half of the deflection at saturation, highlighting the importance of considering time-dependent moisture diffusion.