This paper seeks to extend the definition of a sub-centre beyond the usual definition as a place with significantly larger employment density that has an effect on the overall employment density of the nearby locations. Together with the previous conditions, it is suggested that it is necessary to include another which represents a structural element of an urban sub-system within the metropolitan configuration—that is, a place with intense spatial interaction with its hinterland. Therefore, a metropolitan area can be seen as one comprising urban sub-systems characterised by greater or lesser polycentrism. In this paper, a ‘bottom—up’ methodology based on commuter flows is proposed in order to detect metropolitan sub-centres. Using empirical data from the Metropolitan Region of Barcelona, the proposed methodology is tested, comparing its results with those of other commonly used methodologies (cut-offs, parametric and non-parametric models). The results suggest that the proposed methodology permits optimising the sub-centre delimitation process.