The bed stress under a breaking solitary wave runup was investigated in this study using the Simultaneous Coupling Method (SCM). The SCM couples the shallow water equation (SWE) with the k-ω model. The depth averaged velocity from the SWE is applied as the upper boundary condition in the k-ω model for the bed stress assessment from the boundary layer. It was found that the boundary layer approach provides more accurate bed stress estimation than the empirical method, which leads to a more accurate prediction of the runup height and the wave profile. The accumulation of the bed stress during a solitary wave runup was evaluated. The bed stress in the direction of leaving the shoreline will have more impact in the overall process. However, during a short period of the runup process, the bed stress toward the shoreline may have significant effect as well.