The boreal forest is one of Earth’s most climatologically sensitive regions, and changes in the cover and structure of its vegetation pose a positive carbon-climate feedback on atmospheric greenhouse warming. The region has also experienced more than three times climatological warming of any forested biome in recent decades. While ecological models predict a northward shift of boreal tree cover in response to climate change, comprehensive data have not been available to test the hypothesis. Here we report a test of the magnitude, direction, and significance of changes in the boreal canopy based on the longest and highest-resolution record of calibrated satellite maps to date. The boreal canopy increased in density and shifted northward from 1984 to 2020, with the largest and most significant gains in its northern latitudes. Net forest gains occurred despite stable rates of disturbance across all but the region’s southernmost latitudes, implicating widespread release of climatological limitation on growth over changing distribution of fire, harvest, insect, and other disturbances. These new forests will sequester carbon as they mature, increasing its residence time in woody biomass, and will play a key role in how the terrestrial biosphere attenuates atmospheric CO2 increases.