The Global Terrestrial Network for Permafrost (GTN‐P) established by the International Permafrost Association (IPA) and WMO under the GCOS and GTOS networks is the primary international global program for monitoring the permafrost Essential Climate Variables (ECVs). Active Layer Thickness (ALT) is formulated as one important ECV by WMO and GCOS. The presented data collection relies on ALT data collected and managed by the Circumpolar Active Layer Monitoring Network-CALM under GTN‐P / IPA. The CALM observational network observes the long-term response of the active layer and near-surface permafrost to climate change over multi-decadal time scales in the Northern and the Southern hemisphere. The CALM activities and data are hosted on the CALM website http://www.gwu.edu/~calm. This presented data collection contains 263 ALT time series located in the Northern Hemisphere and spans 34 years (1990-2023) of annual (discontinuous) ALT. The ALT time-series cover the Arctic and mountain permafrost in the Swiss Alps and Asia, latitudinal and elevation gradients, making these records unique and providing an important snapshot of the state of permafrost across these complex landscapes. The data provided here has been assembled from the original data sources and optimized by the validation team in the ESA Permafrost_cci project for the validation of the Permafrost_cci ALT time series maps.