By the time the Turkish Republic was founded in 1923, Istanbul had become a marginalized city. A series of wars and devastating fires during the final years of the Ottoman Empire took a massive toll on the city’s economy and physical landscape, which posed the necessity of a comprehensive urban renewal. This dissertation traces the circulation of ideas, theories, and proposals about the modernization of Istanbul’s built environment during the early republican period (1923-1949). Focusing on urban planning, architecture, and historical preservation, it offers an urban history of Istanbul at this critical, post-imperial juncture that coincided with the rise of nationalism and global modernism. This dissertation departs from the standard historiography that overemphasizes the agency of the nation-building program and its top-down state engineering in the social and spatial reconfiguration of Turkey. Instead, I argue that an entangled web of global, national, and local dynamics fueled the urban transformation of early republican Istanbul. Drawing on archival material and contemporary publications, I specifically focus on the crucial but hitherto neglected role of local agents in shaping the politics and practices of urbanism. By examining particular moments of urban intervention (such as the opening of boulevards, the redesigning of public squares, and the demolition of historical monuments) and the public debates that surrounded them, this dissertation demonstrates how Istanbul-based institutions and individuals transformed their city by navigating through the theories of modernism, the nationalist ideology, and their own diverse visions of modernity and urban modernization.