Rainfall-induced landslides in volcanic ashes represent a major natural hazard in many regions around the world. Owing to their loose structure, volcanic ash slopes are prone to rainfall-induced landslides. The paper presents a continuum modelling approach for the analysis of wetting-induced instability phenomena at the onset of failure in loose volcanic ash slopes. A numerical simulation of a landslide-prone volcanic slope in Costa Rica is carried out with a two-dimensional hydro-mechanical finite-element slope model. A constitutive model based on the effective stress concept extended to partially saturated conditions is used to reproduce the volcanic ash hydro-mechanical behaviour. The model parameters were calibrated through a previous extensive laboratory testing campaign. Simulation results allow the behaviour of the slope during rainfall infiltration to be anticipated and the development of the failure mechanism to be analysed. Results of the coupled numerical model demonstrate the important role of wetting and drying cycles, slope geometry and bedrock on the timely evolution of matric suctions and wetting-induced deformations. Moreover, it is observed that the pore collapse upon wetting enhances the development of a localised shear failure mechanism in unsaturated conditions.