The effects of interstitial fluid on the behaviors of two approaching particles play an important role in the rheology of granular–fluid mixtures. In this paper, an Euler–Lagrange model is developed to study the effects of interstitial fluid on the frictional behaviors of particle contacts which is resolved by the discrete element method (DEM). A formulation of the friction coefficient for immersed particle contacts, involving the effects of interstitial fluid viscosity and pressure, is proposed based on the results of a laboratory experiment. The enhancement effect of the porosity of granular materials on the inter-particle friction coefficient is taken into account. For the inter-particle normal contact force, an existing empirical formula of the coefficient of restitution for immersed particle collisions is adopted. The model is verified in the laboratory experiment of collisions between a single particle and a solid wall in both air and water. The model is further validated by simulating subaerial granular landslides into water down an inclined slope. Without the effects of interstitial fluid on the inter-particle friction coefficient, the propagation velocity of subaerial landslides intruding into the water and the generated tsunami wave is underestimated. Moreover, if the enhancement effect of the landslide porosity is excluded, the propagation velocity of the landslide and the generated wave are overestimated.