The present study investigates the phenomenon of spatial segregation and the formation of a dual geography within the urban fabric of Qajar-era Tehran, focusing primarily on the developmental process of the Foreigners’ Quarter (or the European sector) of the capital. The fundamental objective of this article is to elucidate how the integrated and organic spatial organization of the city collapsed in the face of imported Western urban planning models and foreign diplomatic influence. In the pre-modern era, the distribution of wealth and the settlement of elites in the capital followed an indigenous and coherent logic. However, relying on a sociological analysis of historical travelogues, the present research demonstrates that this physical integration was ruptured by the arrival of European diplomats and the subsequent consequences of colonial rivalries. The physical construction of the new sector was a gradual process, beginning with the scattered establishment of embassies and diplomatic suburbanization in the northern summer resorts, which ultimately culminated in the formation of a distinctly separate geographical zone in the north of the city during the Naseri era. The construction of axes such as the "Ambassadors’ Boulevard"—characterized by wide streets, sidewalks, orderly tree plantings, and right-angled intersections—created a modern neighborhood exclusively designed for the isolated lifestyle of diplomats and Western-oriented aristocrats. Consequently, this spatial intervention resulted in a profound social and physical segregation; while the European quarter transformed into a showcase of modern amenities, tramways, and extroverted architecture, the old sector of the city remained plagued by dense overcrowding, unpaved alleys, polluted waterways, and acute infrastructural crises.