ABSTRACT A growing interest exists in robust tools to evaluate urban environmental performance. This study introduces a Green City Index framework, designed to assess and compare the sustainability of urban districts. Istanbul, with its significant population density and economic influence, serves as the pilot city in Türkiye. The study included (i) examining existing city indices, (ii) identifying indicators for environmental quality (EQ), community welfare, and DS, (iii) collecting and producing relevant data, and (iv) establishing the Green City Index using the Analytic Hierarchy Process. The highest‐scoring district in Istanbul achieved only 59 out of 100, indicating that no district's environmental performance is sufficiently strong. Even in well‐managed districts, EQ was lacking. A significant relationship between the total index and other subcategories was observed. Recognizing index development as dynamic, this Green City Index provides decision‐makers with a tool for strategic urban management, helping them assess and improve sustainability efforts in line with long‐term goals.