The Terrestrial Multidisciplinary distributed Observatories for the Study of the Arctic Connections (T-MOSAiC) 2021 permafrost thaw data set provides standardized measurements for permafrost thaw and the linked parameters snow depth, vegetation height, water level and soil texture as suggested by Boike et al., 2021. It contains measurements from eight circumpolar permafrost sites where data was obtained using the myThaw app which is based on the user-friendly, multiparameter protocol for standardized monitoring of permafrost thaw for the year 2021. The data set contains detailed metadata for each site including data on the timing of data collection, geographical coordinates and land surface characteristics (vegetation, ground surface and water features).