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Traditional sacrificial concrete shear keys are generally employed in highway bridges to restrain the excessive transverse displacements of the bridge superstructures and protect the substructures from being seriously damaged under the earthquakes. However, the post-earthquake rehabilitation or reconstruction of the conventional concrete shear keys are difficult and inconvenient. To this end, this paper proposed and designed a novel modified post-tensioned unbonded prestressing ultra-high-performance-concrete (UHPC) prefabricated retaining block (UHPC-PRB) structure with the straight joint connection. The novel UHPC-PRB structure consisted of a prefabricated UHPC retaining block, a prestressing anchorage system, and a cast-in-situ cap beam. The prefabricated UHPC retaining block was installed to the cast-in-situ cap beam using the unbonded prestressing tendons. The proposed UHPC-PRB structure is supposed to be effective in not only providing the same functionality as that of the traditional concrete shear keys, but also is expected of simplifying the retrofit procedure and reducing the rehabilitation expense simultaneously. To investigate the seismic behavior of the proposed UHPC-PRB structure, four test specimens were designed and the pseudo-static tests were carried out. The damage process, displacements, strains, and stress states of the specimens were studied. In addition, the influences of the loading height, thickness of the UHPC retaining block, and the initial tension of prestressing tendons on the seismic performance of the proposed UHPC-PRB structures were also examined. Finally, the experimental results indicated that, (i) the proposed UHPC-PRB structures had excellent lateral deformation capacity and self-resetting ability; (ii) the initial tension of prestressing tendons had significant effects on the lateral displacement, critical rotational load, and horizontal load-carrying capacity of the UHPC retaining blocks; (iii) the prestress loss of prestressing tendons had certain effect on the development of the bilinear analytical model of the UHPC retaining blocks, but this effect could be ignored in practical highway bridges; and (iv) increase of the horizontal loading height could reduce the critical rotational load and load-carrying capacity of the UHPC retaining blocks. [ABSTRACT FROM AUTHOR]