Ottoman garden design was shaped by cultural interactions along the Turks' journey from Central Asia to Istanbul and by the Byzantine heritage. From the conquest of Istanbul to the Tulip Era, simple and informal arrangements dominated, while Western influences became prominent during the Tulip Era. In the reign of Selim III, Renaissance and Baroque-style formal gardens became widespread, and by the late 19th and early 20th centuries, the English naturalist style was adopted. However, Western influences did not alter the traditional character of Ottoman gardens. During the Ottoman period, Istanbul’s gardens took various forms depending on their function and associated structures, including palace, pavilion, mansion, waterfront residence gardens, public and foreign institution gardens, religious gardens, parks, groves, and recreational areas. Today, Istanbul's historical gardens represent a rich cultural heritage in both aesthetic and functional terms. However, these gardens have suffered damage over time due to various factors, and future threats to their preservation remain. To contribute to the preservation of these gardens, this study aims to assess their current preservation states and sustainability. In line with this objective, the gardens were classified based on their preservation state, and through literature research and field studies, their present condition, existing threats, and conservation policies were evaluated. Over time, many gardens have been destroyed or erased due to misuse, neglect, improper interventions, uncontrolled urbanization, and environmental factors. These gardens can be classified as “partially preserved”, “destroyed” or “erased” based on their preservation state. Today, palace and pavilion gardens are among the best examples of partially preserved gardens. Gardens of foreign institutions have retained their original structures due to diplomatic protection. Waterfront residence gardens remain partially preserved due to their prestige, while mansion gardens have been largely destroyed or erased by urbanization. Although religious building gardens were generally preserved, landscape renovations have altered their original designs. Public building gardens suffered severe damage from additional construction and neglect. Recreation areas and parks were mostly destroyed or erased, while groves, though spatially protected, have deteriorated due to neglect and construction pressure. This situation necessitates the implementation of more effective measures to ensure the preservation of historical gardens for future generations. If the current threats persist, the transmission of these gardens as cultural heritage will become increasingly difficult. Although successful conservation and restoration efforts have been carried out in palace and pavilion gardens today, mansion gardens, parks and recreation areas, face significant preservation risks due to misuse, neglect, improper interventions, and climate change. These risk factors, which make it increasingly difficult to preserve historic gardens in the future, gradually deteriorate their physical integrity and endanger the transmission of cultural heritage to future generations. Although Turkey has adopted international agreements and developed its national legislation accordingly for the protection of historic gardens, implementation deficiencies such as the lack of conservation projects, qualified workforce shortages, and the absence of expert supervision persist. For the sustainability of historical gardens, comprehensive conservation policies that reduce construction pressure, preserve the original structure, and minimize the effects of climate change must be implemented.