AbstractGrowing evidence shows that lithospheric mantle beneath cratons may contain a certain amount of water that originated from dehydration of subducted slabs or mantle metasomatism. As water can significantly reduce the viscosity of nominally anhydrous minerals such as olivine, hydration‐induced rheological weakening is a possible mechanism for the lithospheric thinning of cratons. Using 2‐D thermomechanical numerical models, we investigated the influence of water on dislocation and diffusion creep of olivine during the evolution of cratonic lithosphere. Modeling results indicate that dislocation creep of wet olivine alone is insufficient to trigger dramatic lithospheric thinning within a timescale of tens of millions of years, even with an extremely high water content. However, if diffusion creep is incorporated, significant convective instability will occur at the base of the lithosphere and drive lithospheric mantle dripping, which results in intense lithospheric thinning. We performed semianalytical models to better understand the influence of various parameters on the onset of convective instability. The convective instability promoted by hydration weakening drives lithospheric mantle dripping beneath cratons and thus provides a possible mechanism for cratonic thinning.