Monitoring ice-marginal lakes is important for glaciological and geomorphological studies, as well as hazard and risk assessment. Sentinel-1 synthetic aperture radar (SAR) and Sentinel-2 optical data opened a new era for multi-temporal analysis and studying geomorphological changes. The purpose of this study is to compare the applicability of Sentinel-1 and Sentinel-2 data for mapping the changes in ice-marginal lake areas at the southern margin of the Vatnajökull ice cap, southeast Iceland, between 2016 and 2020. We semi-automatically mapped the ice-marginal lakes with object-based image analysis (OBIA) and based on image time series using 1) the polarization products derived from Sentinel-1 data, and 2) the spectral information of Sentinel-2 data, and compared the results. Our results show that Sentinel-1 performed better regarding the detection of the number of ice-marginal lakes, whereas Sentinel-2 performed better regarding lake delineation. Moreover, we discuss the applicability of optical and SAR data for mapping and monitoring the evolution of ice-marginal lakes.