The aim of this work is to establish a correlation between water uptake by anhydrous sodium naproxen (ASN) at two different relative humidities and modifications in tableting and densification behaviour under hydration. Water uptake was evaluated at different relative humidities. Models for the hydration kinetics of ASN at 55% and 86%, corresponding to the formation of the dihydrated and tetrahydrated forms, respectively, were evaluated assuming Eyring's dependence on temperature. Tabletability, compressibility, compactibility, and densification behaviour were determined using an instrumented single punch tablet machine. Kinetic data are consistent with a model where water molecules enter the crystal preferentially along hydrophilic tunnels existing in the crystal structure and corresponding to the propionate side chain. Water inclusion perturbs the crystallographic structure, causing slight structural changes according to the amount and associated to an increase in entropy. The interposition of water molecules between sodium naproxen molecules weakens intermolecular bonds, and these sites can behave like sliding planes under compression. Such structural changes may explain the improved compression behaviour and modified densification propensity mechanism. Kinetic data describing the water hydration mechanism of ASN explain in an original way the improved tableting and densification properties under hydration.