<p>Elevated reinforced concrete (RC) water tanks are highly susceptible to seismic loading due to hydrodynamic sloshing effects, which can significantly increase structural responses and compromise operational safety. This study presents a nonlinear fluid–structure interaction (FSI)-based numerical investigation of the seismic behaviour of an elevated RC circular water tank, with particular emphasis on the effectiveness of baffle walls in mitigating sloshing-induced demands. Three-dimensional dynamic simulations were performed in ABAQUS using a coupled Eulerian–Lagrangian (CEL) framework, incorporating material nonlinearity and realistic interaction between the contained water and the tank structure. The 2001 Bhuj earthquake ground motion was applied as horizontal base excitation to evaluate the tank response under half-filled conditions for configurations with and without baffle walls. The seismic performance was assessed in terms of fluid displacement, velocity, acceleration, structural displacement, base shear, and S11 stress distribution. The results demonstrate that the incorporation of baffle walls significantly suppresses sloshing activity and improves the overall seismic response of the tank. Compared with the unbaffled configuration, the baffled tank exhibited reductions of approximately 83% in maximum water displacement, 50% in fluid velocity, and 44% in fluid acceleration. Furthermore, the base shear was reduced by approximately 36%, while the S11 stress distribution showed lower stress concentrations and a more uniform stress pattern in the tank wall and staging system. Structural displacement, velocity, and acceleration responses were also significantly reduced in the presence of baffles.</p>