The effects of interstitial water on the collapse of partially immersed granular columns are experimentally and numerically investigated. Experiments on the collapsing process of partially immersed granular columns over a horizontal bed are conducted in which the saturation level of the columns, H, is defined by the ratio of the initial interstitial water level height to the total height of the column. It is shown that the interstitial water generally speeds up the collapse if the column consists of coarse-grains but slows down the motion if the column consists of fine-particles. The final run-out of a coarse-grain column increases as the saturation level increases, while that of a fine-particle column first decreases as the saturation level increases until H = 0.75 and then increases to a value still smaller than the final run-out in the relevant dry case. In the experiments, the drag force between the water and the particle phases seems to always accelerate the collapse of partially immersed columns. It is speculated that there is an additional inter-particle viscosity in the water-particle mixtures, which retards the collapsing processes and dominates over the drag force in fine-grain cases. In the present study, a particle-fluid two-phase model is then adopted to describe the collapsing dynamics of partially immersed granular columns in which the water-air interface in the granular mass is captured and the primary flow variables of both particle and water phases are resolved. The model is validated by the experimental results. The roles of the drag force and the pressure gradient force exerted on the particles by the interstitial water are then clarified. It is shown that the interphase drag and the pressure gradient force drive both coarse- and fine-grain columns to collapse, and their effects increase with an increase in the saturation level of the column. The significance of the additional inter-particle viscosity induced by the interstitial water is also discussed.