The mechanical properties of wood are complex not only because of its viscoelastic and orthotropic nature, but also due to the influence of moisture content variations. This work characterizes the mechanical and diffusion properties of the S2 layer of the wood cell wall through finite element simulations of nanoindentation using a finite-strain hygro-viscoelastic rheological model. The methodology consists of simulating nanoindentation tests at different relative humidity levels and calibrating material parameters against experimental measurements. The model is formulated within a hydrogel-based framework in which the wood matrix is described as a polymeric gel reinforced with cellulose fibrils. The kinematics assume a multiplicative decomposition of the deformation gradient into elastic, viscous and swelling contributions. Three rheological branches (elastic-swelling, matrix viscoelasticity and fibril viscoelasticity) are combined in a generalized Maxwell arrangement. Contact is incorporated via a penalty term added to the variational formulation. The numerical implementation employs a monolithic three-field finite element formulation, with displacement, chemical potential, and pressure as primary fields. Taylor-Hood type interpolations are used to satisfy the inf–sup stability condition. Weak forms of equilibrium, mass conservation, and chemical potential balance are derived, while internal variables evolve according to nonlinear differential equations. The resulting system is solved using the finite element software FEniCSx. After validation, force-displacement curves are extracted by inverse analysis based on the calibrated properties. The proposed framework provides a thermodynamically consistent and numerically stable basis for characterizing coupled phenomena in wood and other natural fiber-matrix composite materials.