In 1966, the government-sponsored Plan Organization commissioned the first Tehran Master Plan (TMP), setting the stage for the Iranian capital’s extensive transformation and its spatial and social re-structuring. In the process, Abdolaziz Farmanfarmaian, an Iranian architect, collaborated with Victor Gruen Associates of the United States to implement lessons from Gruen’s urban model, the ‘Cellular Metropolis of Tomorrow’, in a reorganization of the socio-spatial structure of Tehran. After a three-year study of the city’s socio-economic and physical organization, the planners proposed decentralization of the congested old city centre and the development of new ‘modern’ centres of activity along a rapid transit line extending west-ward. This article engages with ongoing discourses on inclusive cities and reflects upon the segregating effects of boundary-edges to argue that the planners’ emphasis on locating urban facilities in the centre of communities led them to ignore emerging dead edges between socially divided neighbourhoods, ultimately hindering social interactions. The allocation of these edges to urban infrastructure, highways, urban voids, and large green spaces isolated and insulated each community from the larger urban area. Through the analysis of the TMP’s reports, this article reveals how the modernist planners mediated the creation of socio-physical boundaries, segmented the city, and increased social exclusivity.