Abstract In previous studies of the structural behavior of castellated steel beams, different possible failure modes of these extensively used structural members have been identified and investigated. On the other hand, during the past 25 years or so, a proliferation of research work has been undertaken on the distortional buckling of steel members. Nonetheless, no studies are found in the literature on the distortional buckling of castellated beams. Accordingly, tests of six full-scale castellated beams are described, in which the experimental investigation of distortional buckling was the focus of interest. In addition to the test strengths, the accurate critical loads of the beams have been obtained using some extrapolation techniques, and ultimately a comparison has been made between the obtained test loads and some theoretical predictions.