Delineating the distribution of tidal flats in the Yangtze River Delta region is of considerable significance for the ecological environment research and sustainable development of the region. However, the tidal flat ecology is greatly threatened by rapid economic development and human disturbance. Information on the tidal flat ecology is necessary for tidal flat extraction studies in Jiangsu Province and Shanghai City. Therefore, using imagery acquired by Sentinel-2 sensors along the coasts of Jiangsu Province and Shanghai City and the Google Earth Engine Cloud Platform, we established a multilayer automatic decision tree classification model based on the maximum spectral index composite algorithm and Otsu algorithm to extract tidal flats. A total of 1011 scene images, acquired from January 1 to December 31, 2021, were collected to generate tidal flat maps of Jiangsu Province and Shanghai City. The overall accuracy obtained by establishing the confusion matrix was 96.1%. The tidal flat areas of Jiangsu Province and Shanghai City in 2021 were 222347 and 41097 hectares, respectively, among which the tidal flat area of Jiangsu Province was 5.4 times that of Shanghai City. Moreover, we compared the results with real images and existing products, which further explained the rationality of the tidal flat results of this study. The 10-m-resolution tidal flat map of Jiangsu Province and Shanghai City obtained in this study can provide support for the sustainable policy management of tidal flats in the Yangtze River Delta region.