The effect of high-pressure (HP) densification (30, 90, and 150 MPa for 3, 30, and 300 s) on the moisture sorption properties of Paulownia wood was investigated. After the densification, samples were conditioned at three temperatures (20, 30, and 40 °C) and five equilibrium moisture contents (from 11.20 to 95.62%) during the study, after which the equilibrium moisture contents of the control and treated samples were measured. The HP-treated groups had higher equilibrium moisture contents than the controls at higher relative humidity levels. The hysteresis phenomenon and the scanning electron microscopy observations were explained by the transformation of the structural elements by the HP treatment. Finally, two moisture sorption isotherm (MSI) models (a linear polynomial model for adsorption and a quadratic polynomial model for desorption) were established with good performance to describe the relationship between HP treatment parameters, environmental conditions, and equilibrium moisture contents.