Abstract: The coordinated low-carbon development of urban agglomerations plays a vital role in coping with climate change and achieving the ‘dual carbon goals’. Assessing the level of regional low-carbon development efficiency and understanding its spatiotemporal evolution is of great significance to promoting the coordinated low-carbon development of urban agglomerations. Therefore, this article establishes a low-carbon development efficiency measurement method to evaluate the low-carbon development efficiency of the Yangtze River Delta urban agglomeration from 2005 to 2020 from three dimensions: economic, social, and ecological. On this basis, low-carbon development zoning in the Yangtze River Delta region was realized. The results demonstrate that: (1) The calculation method constructed effectively evaluates the low-carbon development efficiency level of each dimension within urban agglomerations, thereby obtaining a comprehensive efficiency evaluation of low-carbon development. (2) The comprehensive efficiency of low-carbon development shows the spatial distribution characteristics of ‘high values clustered at multiple points and low values concentrated in contiguous areas’. During the entire study period, high-value cities shifted from being clustering in central Anhui Province, northern Anhui Province, and southern Zhejiang Province to clustering in the Shanghai-Nanjing-Hangzhou-Hefei-Ningbo Development Belt. On the contrary, low-value cities are concentrated in the central region of the Yangtze River Delta and are distributed in a strip. (3) Based on calculation of low-carbon development efficiency of cities in the Yangtze River Delta, the cities in the Yangtze River Delta can be divided into five types of low-carbon development advantage zones. These include economic low-carbon advantageous zone, social carbon sharing advantageous zone, ecological carbon sink advantageous zone, economic and social low-carbon advantageous zone, and social ecological low-carbon advantageous zone. The study has revealed significant spatial differences in the efficiency levels of different aspects of low-carbon development in the Yangtze River Delta urban agglomeration. It is crucial to propose differentiated strategies tailored to specific characteristics of each zone´s low-carbon development and promote coordinated green and low-carbon growth within urban agglomerations.