<p>To resist loads, a steel–concrete composite bridge structure combines a steel profile and a concrete deck. When designing these structures, the shear lag effect and the concept of effective width are essential parameters in the final strength of the member. Although the analysis of effective width at ambient temperature is well understood, its effectiveness in fire situations remains uncertain. Few studies have investigated the effective width in steel–concrete composite box girder bridges. Moreover, no research has been identified that examines the effective width under fire conditions while accounting for all the previously mentioned parametric variables. Accordingly, this study aims to evaluate the influence of fire on the effective width of composite box girders. Nine thermo-structural models were developed in Abaqus/CAE^® to study the impact of fire on the effective width of steel–concrete composite box girder bridges. The models were calibrated through two separate validations with experimental tests from the literature. Subsequently, a parametric analysis of the models was conducted by varying the following parameters: slab height, distance between webs, longitudinal span, and fire exposure time. It was found that the effective width is sensitive to all the parametric variables. A comparison of the effective width under fire conditions with that at ambient temperature revealed a significant reduction in effective width during the first 30 min of fire exposure. Based on the analysis of these results, we propose a procedure for determining the effective width in fire situations using reduction coefficients applicable to both the AASHTO and EN 1994-2 standards.</p>