Steel and steel-concrete composite plate girders are prevalent structural components extensively employed in the design of load-bearing structures with substantial capacities or spanning considerable distances. Despite lateral stiffeners reinforcing the slender webs of plate girders, these members remain susceptible to out-of-plane shear buckling. This susceptibility is further exacerbated when subjected to elevated temperatures, particularly in the event of a fire. However, the comprehensive investigation of the shear buckling behaviour of steel and steel-concrete composite plate girders, both under normal ambient conditions and at elevated temperatures, remains a topic that warrants further exploration. This study is dedicated to experimental and numerical investigations of shear buckling phenomena of steel and composite plate girders under fire exposure. At first, large-scale elevated temperature tests on composite plate girders in shear buckling are presented, and results are disseminated to describe the structural fire response of these girder types. This study is followed by the formulation and validation of a computational framework for scrutinizing the phenomenon of web shear buckling in plate girders under elevated temperature conditions, leveraging the ABAQUS software. This is done by direct validation of the proposed numerical model against the experimental findings. Subsequently, this validated modelling approach is harnessed to create a benchmark plate girder, on which a parametric analysis is carried out, targeting pivotal parameters influencing web shear buckling, including web slenderness, panel aspect ratio, degree of shear connection, load ratio, heating scenario and fire protection. Our contribution draws the following initial conclusions: (a) higher web slenderness, higher panel aspect ratio, higher load ratio and partial shear connection lead to a lower overall fire resistance and (b) concrete slab stabilizes and increases the stiffness of the top flange and acts as an insulator to provide a slower heating of the top flange.