The dynamic nature of current competitive environments has motivated managers to identify sources of flexibility based on the organisation's human resources. In this regard, internal labour flexibility emerges as a valid strategy to counteract environmental changes, based on the adaptability of the organisation's current workforce to face non-routine circumstances and events that require creativity and initiative. However, despite recent interest in the notion of internal labour flexibility, there is no commonly accepted definition of this term. Currently, there are several competing definitions of the construct, along with a variety of operationalisations. Some integration of these approaches is needed to ensure that our knowledge of internal labour flexibility grows cumulatively. In this study, we have sought to address some of these limitations by theoretically conceptualising and empirically validating a scale to measure internal labour flexibility from the resource-based view (RBV). The application of structural equation modelling to a sample of 226 Spanish firms confirmed the suggested dimensionality of the internal labour flexibility scale, which distinguishes between employees' intrinsic flexibility, skill malleability, behavioural malleability and relational flexibility. Tests of reliability and validity confirmed the appropriateness of this measurement scale.