AbstractFlood-induced scour has been identified as a major cause of bridge collapse around the world. Although the seismic response of bridges with scoured foundations has been studied extensively, there is limited information on how seismic isolation impacts the dynamic behavior of bridges that have scoured foundations and experience an earthquake. Although both scour and seismic isolation affect bridge natural frequencies, the combined effect of these two are relatively scarce in the earthquake engineering field. This study explores the consequence of pier scour on the seismic behavior of base-isolated bridges. A base-isolated, multispan, continuous concrete girder bridge with a scoured foundation, located in Western Canada, is considered for this purpose. Two isolation bearing types, a friction pendulum system (FPS) and lead rubber bearings (LRB), are considered in this study. For isolated bridges, the failure probability under the combined action of scour and earthquake hazard is evaluated considering different combinations of seismic and flood hazards at the bridge site. Through extensive nonlinear analysis under combined seismic and scour hazard, this study compares the seismic performance and failure probability of isolated bridges using fragility curves and fragility surfaces. The outcomes lead to more clear identification of the most critical condition for the design of base-isolated bridges subjected to multiple hazards.