The assessment of the seismic vulnerability of existing masonry arch bridges has gained growing attention within the technical and scientific community, particularly in Italy, where these structures hold both critical infrastructural and cultural importance. Their inherently complex structural behavior calls for advanced modeling approaches capable of reliably capturing their response under static and dynamic loads, thereby facilitating the identification of effective retrofitting solutions. This study investigates the seismic performance of masonry arch bridges through the development of three-dimensional discontinuum substructural numerical models using the commercial software 3DEC. The modeling framework incorporates both rigid and deformable blocks, parameterized to represent structural and non-structural components such as piers, abutments, backing, spandrel walls, and filling material. Static and dynamic analyses are conducted to evaluate the response of these components under seismic loading. By employing a substructural modeling approach, it becomes possible to isolate and analyze critical bridge segments, such as vertical sections taken halfway between adjacent arches or halfway between consecutive piers. This approach offers the advantage of significantly reducing computational demands while preserving the accuracy of the simulated structural response. The findings contribute valuable insights for the seismic vulnerability assessment of historic masonry arch bridges, supporting their preservation and enhancing structural resilience.