Bridges located in coastal areas are highly vulnerable to extreme hydrodynamic events such as tsunamis and storm surges, where deck unseating and wash-off represent recurrent failure mechanisms. In current engineering practice for structural assessment, these phenomena are typically addressed through simplified approaches based on equivalent static forces. More recently, research efforts have moved toward the use of CFD simulations to estimate hydrodynamic actions, modeling the bridge deck as a rigid body and subsequently applying the resulting force time histories to a structural FEM model. Although representing a significant improvement, these approaches remain inherently decoupled. The fluid problem is solved independently from the structural response, and the structure is either treated as rigid or subjected to prescribed hydrodynamic forces. As a result, key aspects of fluid–structure interaction are not captured: the kinematic interaction, since the fluid velocity and pressure fields are unaffected by structural deformation, and the inertial interaction, since only force time histories are applied, neglecting water mass inertia and structural feedback. Consequently, true two-way FSI effects are not represented. This work presents a fully coupled FSI modeling strategy for simulating bridge deck wash￾off under tsunami-like actions within OpenSeesPy. The approach leverages the recent implementation of the Particle Finite Element Method (PFEM) in OpenSees. Owing to its Lagrangian formulation and natural compatibility with structural FEM discretization, PFEM enables a monolithic treatment of fluid and structural domains, ensuring consistent kinematic and inertial coupling. The methodology is demonstrated through two-dimensional and three-dimensional simulations of simply supported bridge decks subjected to extreme hydrodynamic loading. The results highlight how accounting for two-way FSI significantly influences the prediction of uplift, sliding, and eventual wash-off when compared to traditional equivalent-force and CFD-derived pressure time histories approaches. The proposed framework provides a physically consistent and computationally accessible tool for assessing coastal bridge structures subjected to tsunami events within a widely adopted structural analysis platform, supporting more reliable performance-based evaluations in hazard-prone regions.