For　a　bridge　located　in　a　seismically　active　and　flood-prone　region,　the　occurrence　of　earthquakes　combined　with　flood-induced　scour　is　a　highly　possible　multihazard　event.　This　study　quantifies　the　scour　effect　on　the　seismic　performance　of　a　single　pylon　cable-stayed　bridge　under　bidirectional　earthquake　excitations.　Three-dimensional　finite-element　models,　considering　the　nonlinear　soil-structure　interaction,　the　flexure-shear　behavior　of　the　bridge　pier,　and　the　hydrodynamic　force　applied　to　bridge　structures,　are　built　on　the　OpenSees　platform.　Several　scour　depths　are　considered　in　this　study　to　represent　various　scour　intensities　within　the　service　life　of　the　cable-stayed　bridge.　The　modal　analyses　indicate　that　the　periods　of　the　few　vibration　modes　(fourth　and　fifth　modes)　of　the　cable-stayed　bridge　increase　as　the　scour　progresses,　and　the　scour　effects　gradually　diminish　for　higher　and　lower　vibration　modes　(first,　second,　and　third　modes).　The　results　of　the　nonlinear　time　history　analyses　indicate　that　the　moments　on　the　piles　increase　as　scour　progresses,　while　the　moment　demand　of　the　pier　decreases.　The　moment　in　the　pylon　of　the　scoured　bridge　is　much　larger　than　the　intact　bridge　due　to　the　higher　pounding　potentials　with　the　increase　of　scour　depth.　Obviously,　the　failure　modes　of　the　cable-stayed　bridge　transform　from　the　pier　to　the　piles　and　the　abutments.　In　addition,　the　impact　of　the　hydrodynamic　force　on　the　seismic　response　of　the　bridge　is　negligibly　small.　The　findings　of　this　study　can　be　used　to　guide　the　seismic　design　and　the　retrofitting　of　cable-stayed　bridges　under　scour-critical　conditions.