AbstractDue to their thin‐walled nature and complex geometry, cold‐formed steel purlins are prone to a variety of instabilities. In most applications, purlins rely on the attached sheeting to restrain member buckling and hence their cross‐section is optimized to maximize their cross‐sectional resistance to local and distortional buckling caused by sagging bending moments under the action of a uniformly distributed load. Sigma purlins are a family of cold‐formed steel members with folding‐lines along their webs. The non‐straight geometry of their web is beneficial in terms of reducing the local slenderness of the web, however it increases their susceptibility to web crippling in case the purlins are subjected to concentrated transverse loads and hence it may reduce the overall moment resistance. This study reports a series of experimental tests on sigma purlins interior‐one‐flange (IOF) loading condition. The tested specimens employ two different section geometries and three different bearing plate widths. The test results show that all the specimens failed by web crippling, however two distinct failure modes and corresponding load‐deformation curves are observed depending on the section geometry. To investigate further the effect of web geometry on the IOF web crippling strength of sigma sections, an FE model was developed and validated against the reported test results. Following successful replication of the experimental observations, a comprehensive parametric study was performed and several sigma sections were numerically tested. Furthermore, the applicability of a novel slenderness‐based design approach previously proposed by Duarte and Silvestre for the web crippling design of channel sections is assessed and design recommendations are proposed.