<p>Staggered widening refers to a rehabilitation scheme in which new bridge piers are not aligned with existing ones due to constraints from current river embankments or road boundaries. To date, research on staggered widening bridges remains limited. Owing to the staggered span layout, girders are supported by piers of adjacent bridges at the mid-span region, leading to a vehicular load distribution mechanism distinct from that of conventional widening bridges, especially for interior girders near the closure slab. This study investigates the load distribution factor (LDF) for bending moment in staggered widening bridges. First, a simplified analytical model is developed using the rigid-jointed girder (RJG) method to derive transverse influence lines. The boundary conditions in the conventional RJG method are modified to capture the mid-span supporting effects provided by adjacent piers. The governing equation of the analytical model is derived based on the updated model configuration. The accuracy of the simplified model is then verified against finite element analysis results. Transverse load distribution characteristics of staggered widening bridges are compared with those of conventional widening schemes. On this basis, a parametric study is performed to examine the effects of key parameters, including diaphragm rigidity, flange rigidity, girder rotational stiffness, closure slab width, girder width, and the number of girders, on the LDF. Finally, based on the parametric analysis results, a generalized formula is proposed for predicting the girder LDF of staggered widening bridges.</p>