Castellated beams, characterized by their unique perforated web design, have gained significant popularity in the construction of industrial buildings and medium-span structures. They offer improved structural efficiency by increasing the overall depth of the beam without adding material. This results in higher bending moment capacities and increased stiffness. In addition, the openings can be used to incorporate building services such as electrical cables and piping, reducing the need for additional space and contributing to more efficient and cost-effective designs. Despite these advantages, the use of castellated beams creates challenges in terms of stability and load-carrying performance. While the web openings increase material efficiency, they can also act as stress concentrators, potentially affecting the overall stability and local buckling. As only the strong axis moment of inertia is increased, while the torsional constant is even decreased around the opening, the section slenderness for the weak axis of castellated beams is comparably higher than the slenderness of regular hot rolled profiles, furthermore increasing the risk of the beams for failure due to global buckling. Consequently, a thorough understanding of their structural performance is essential for optimizing their use in structural applications. This study investigates the stability behaviour and load-carrying capacity of castellated steel members and compares the member’s structural performance with that of regular hot-rolled profiles. To accurately capture the performance, experimentally validated numerical models were used to perform geometric and material nonlinear analysis with imperfections (GMNIA). Critical buckling modes, deflection shapes, and failure mechanisms were analysed and compared for both member types, with the main focus on the influence of different real-world boundary conditions on the load-carrying behaviour. The findings provide valuable insight into the efficient use of castellated beams in steel structures, combining their higher load-carrying capacity with the arising stability challenges due to their unique geometry.