The seismic stability of underground structures, particularly tunnels, is one of the most challenging areas of geotechnical earthquake engineering. Groundwater plays a pivotal role in altering the distribution of internal forces within tunnel linings by influencing pore water pressure, effective stress, and soil stiffness. This study aims to investigate the effect of different groundwater levels on the shear force and bending moment of circular tunnel linings under both static and dynamic conditions. To this end, numerical modeling was conducted using the finite element method via PLAXIS 2D software, and the triple interaction among soil, structure, and pore fluid was analyzed across five distinct groundwater levels and four tunnel embedment depths. The results indicate that rising groundwater levels lead to a significant reduction in shear force and bending moment within the tunnel lining, such that under dynamic conditions, the shear force at the tunnel crown decreases by up to 55% and the bending moment by up to 40% compared to dry conditions. It was also found that under static conditions, groundwater level variations have a negligible effect on internal forces, whereas under dynamic analysis, a drop in groundwater level is accompanied by an increase in shear force. The most sensitive points of the tunnel lining with respect to internal force variations were identified as the lateral sides (left and right springlines) of the tunnel. The findings of this research can be utilized by engineers and designers in the seismic design of tunnels located in areas with high water tables and saturated soils.