Residual stresses induced by welding have a considerable influence on the load-carrying capacity, stability behaviour, and fatigue resistance of welded steel I-section beams. Experimental techniques for residual stress measurement are generally costly and difficult to implement on a large scale, whereas finite element modelling (FEM) requires complex input parameters and high computational effort. This paper presents a new analytical model for predicting the residual stress distribution in welded I-section beams fabricated using fillet welds with different sizes. The proposed model is formulated based on force and moment equilibrium equations, in which the tensile residual stress at the flange-web intersection is assumed to reach the yield strength, the transition zone is represented by higher-order functions, and the compressive residual stress is determined from equilibrium conditions. The comparison results indicate that the analytical model shows good agreement with FEM simulations and accurately captures the experimental trend, particularly within the tensile-compressive transition zone of variously sized welds. Compared with conventional residual stress models, the proposed model provides improved accuracy and general applicability while remaining simple and practical for engineering design applications. Using the predicted distributions as inputs to finite element analyses, residual stresses were found to reduce the ultimate flexural resistance by up to 3.9%, the reduction increasing consistently with weld size. The model therefore offers an efficient tool for predicting welding-induced residual stresses and assessing their influence on the flexural resistance of welded steel beams