Recently, the application of Tunnel Boring Machine (TBM) tunnels has increased in urban areas due to the expansion of underground transportation systems. TBM tunnels are constructed using precast concrete segment linings, in which the mechanical behavior of segment joints plays a critical role in their seismic performance. This study evaluates the nonlinear seismic performance of an operating TBM tunnel constructed in urban soil deposits, with a focus on realistic modeling of segment joint behavior. A two-dimensional coupled numerical analysis considering soil deformation and groundwater flow was performed using a continuum ground model. The segment lining was modeled with beam elements, while the nonlinear rotational and shear behavior of segment joints was represented using the Janssen joint model. The results show that, under undrained conditions, groundwater flow has a negligible effect on tunnel behavior when the lining remains undamaged. Maximum bending moments and shear forces occurred near the crown but remained well below the structural capacity, and joint rotation angles were far below allowable limits. These findings confirm the effectiveness of nonlinear seismic analysis incorporating realistic segment joint modeling for operating TBM tunnels.