Polyamine is a novel inhibitor applied to water-based drilling fluids in order to reduce hydration of clays, especially montmorillonite (MT). In the present study, the water-adsorption properties of montmorillonite intercalated with polyamine were characterized by X-ray diffraction (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and adsorption isotherm tests using water, benzene and heptane. According to the XRD, DRIFT and water-adsorption test results, polyamine (PA)-intercalated montmorillonite (PA-MT) adsorbs less water than either K+- or -intercalated montmorillonite (K-MT, NH4-MT). The surface energy of MT was analysed using the van Oss-Chaudhury-Good (VCG) approach based on the water, benzene and heptane adsorption results. The surface energy between PA-MT and water was less than that of NH4-MT and K-MT. Compared with K-MT and NH4-MT, the surface energy of the Lewis acid portion of PA-MT underwent a more significant decrease, suggesting that the hydration of unexchanged Na+ did not make a discernible contribution. The experimental water-adsorption data were fitted using Langmuir, Brunauer-Emmett-Teller (BET), and Freundlich adsorption models. The Langmuir adsorption models can be used to fit the data for water adsorption on PA-MT across the entire p/p0 range. After plotting the experimental data in the linear format of the Freundlich model, only one linear region for PA-MT was observed, indicating a single water-adsorption mechanism for PA-MT. When compared with PA in the absence of clay, DRIFT spectroscopy showed that the NH2 bending mode for PA-MT shifted from 1571 cm−1 to 1619.7 cm−1. This suggested an increase in hydrogen bonding (H bonding) between the ammonium group and the interlayer water. Across the entire p/p0 range, water is suggested to have adsorbed onto PA-MT through H-bonding interactions between the ammonium group and the interlayer water.