Abstract: Transversely unequal-height pier bents often suffer inconsistent deformation and varying degrees of damage under earthquakes due to stiffness differences. To improve the seismic performance and coordinate the stiffness of unequal-height piers, a cast-in-place reinforced concrete and self-centering rocking hybrid transversely unequal pier bent (RC-SRUP) structural system, composed of a cast-in-place tall pier and a self-centering rocking short pier in parallel, was developed. First, the structural configuration and cooperative working mechanism of RC-SRUP were illustrated, and a simplified calculation formula for the force-displacement relationship is established. Then, specimens with a scale ratio of 1:3 of RC-SRUP and cast-in-place reinforced concrete unequal pier bents (RCUP) were fabricated and subjected to cyclic quasi-static loading tests. The failure modes and hysteretic characteristics of RC-SRUP specimens were investigated, and the failure mechanisms and seismic performance of RC-SRUP and RCUP were compared. The results show that: (1) The loading process of RC-SRUP is divided into four stages, i.e., elastic stage, elastic-rocking stage, plastic-rocking stage, and ultimate stage. (2) The RC-SRUP specimen exhibits flexural damage of the tall pier with no significant damage to the short pier, leading to an ultimate failure mode of tall pier flexural failure. The RCUP specimen shows severe shear damage in the short pier and flexural deformation in the tall pier, with its ultimate failure governed by short pier shear failure. (3) RC-SRUP exhibits symmetric flag-shaped hysteretic characteristics. Compared with RCUP, the ductility coefficient of RC-SRUP increases by 48.6%, and the residual drift decreases by 84.2%. RC-SRUP demonstrates superior ductility and self-centering capability while significantly reducing stiffness degradation. By adopting the rocking system to coordinate the stiffness and strength between unequal piers, RC-SRUP significantly mitigates structural damage, enhances deformation capacity, and demonstrates excellent self-centering capability.