Technical Abstract: Non-point source pollution of fresh waters by agricultural P can accelerate eutrophication of surface waters and limit their use for drinking, recreation, and industry. An important pathway of agricultural P transport is surface runoff, to which unincorporated dung from grazing cattle can be a significant contributor. Computer models commonly used to identify agricultural areas with a high potential for P export do not adequately simulate dung application to the soil surface, dung disappearance, and dung P loss to runoff. We developed a new model to simulate these processes for grazing cattle dung. The model simulates dung organic matter decomposition and assimilation into soil by bioturbation as a function of air temperature and dung moisture. We validated that the model can accurately predict rates of dung disappearance using data from 12 published studies. The model also simulates four pools of inorganic and organic P, P mineralization to water-extractable P (WEP), leaching of dung WEP into soil by rain, and loss of dissolved inorganic P in runoff. We validated the ability of the model to reliably simulate these P processes using data from six published dung P transformation studies and seven runoff studies. Overall, the model represents a novel approach for assessing the environmental impact of grazing dairy and beef cattle.